April 2018 Airstrikes Against Syrian Chemical-Weapons Facilities

(Slip Opinion)

April 2018 Airstrikes Against Syrian
Chemical-Weapons Facilities
The President could lawfully direct airstrikes on facilities associated with Syria’s chemical-
weapons capability because he had reasonably determined that the use of force would be
in the national interest and that the anticipated hostilities would not rise to the level of a
war in the constitutional sense.

May 31, 2018

MEMORANDUM OPINION FOR THE COUNSEL TO THE PRESIDENT

On April 13, 2018, the President directed the United States military to
launch airstrikes against three facilities associated with the chemical-
weapons capability of the Syrian Arab Republic (“Syria”). The President’s
direction was consistent with many others taken by prior Presidents, who
have deployed our military forces in limited engagements without seeking
the prior authorization of Congress. This deeply rooted historical practice,
acknowledged by courts and Congress, reflects the well-established division
of war powers under our Constitution. Prior to the Syrian operation, you
requested our advice on the President’s authority. Before the strikes oc-
curred, we advised that the President could lawfully direct them because he
had reasonably determined that the use of force would be in the national
interest and that the anticipated hostilities would not rise to the level of a war
in the constitutional sense. This memorandum explains the bases for our
conclusion.

I.

On April 7, 2018, the Syrian regime used chemical weapons in the eastern
Damascus suburb of Duma. United States Government Assessment of the
Assad Regime’s Chemical Weapons Use (Apr. 13, 2018) (“USG Assess-
ment ”), https://www.whitehouse.gov/briefings-statements/united-states-
government-assessment-assad-regimes-chemical-weapons-use/. At the
time, the intelligence community had assessed that the regime carried out
this attack with chlorine gas and perhaps with the nerve agent sarin as well.
Briefing by Secretary Mattis on U.S. Strikes in Syria (Apr. 13, 2018) (“Mat-
tis Briefing”), https://www.defense.gov/News/Transcripts/Transcript-View/
Article/1493658/briefing-by-secretary-mattis-on-us-strikes-in-syria/. The
attack, part of a weeks-long offensive by the regime, killed dozens of inno-

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cent men, women, and children, and injured hundreds. USG Assessment. In
this use of chemical weapons, the regime sought to “terrorize and subdue”
the civilian population, as well as opposition fighters. Id.
The Syrian government’s latest use of chemical weapons followed a
string of other chemical-weapons attacks. The regime used sarin in Novem-
ber 2017 in the suburbs of Damascus and in an April 2017 attack on Khan
Shaykhun. Id. It also dropped chlorine bombs three times in just over a week
last spring and launched at least four chlorine rockets in January in Duma.
Id. The U.S. government has assessed that the regime used chemical weap-
ons on many other occasions—it has identified more than fifteen chemical-
weapons uses since June 2017 in the suburb of East Ghutah alone—and
believes that the regime, unless deterred, will continue to make use of such
weapons. Id.
On April 13, 2018, in coordination with the United Kingdom and France,
the United States attacked three facilities associated with Syria’s use of
chemical weapons: the Barzeh Research and Development Center, the Him
Shinshar chemical-weapons storage facility, and the Him Shinshar chemical-
weapons bunker facility. Department of Defense Press Briefing by Pentagon
Chief Spokesperson Dana W. White and Joint Staff Director Lt. Gen. Ken-
neth F. McKenzie Jr. in the Pentagon Briefing Room (Apr. 14, 2018) (state-
ment of Lt. Gen. McKenzie) (“DoD Briefing”), https://www.defense.gov/
News/Transcripts/Transcript-View/Article/1493749/department-of-defense-
press-briefing-by-pentagon-chief-spokesperson-dana-w-whit/. The Barzeh
Center was used for the research, development, production, and testing of
chemical and biological weapons. Mattis Briefing (statement of Gen. Dun-
ford). The Him Shinshar sites were a chemical-weapons storage facility
assessed to be the primary location of Syrian sarin-production equipment, as
well as a chemical-weapons storage facility and an important command post.
Id. In total, the United States launched 105 missiles from naval platforms in
the Red Sea, the Northern Arabian Gulf, and the Eastern Mediterranean.
DoD Briefing (statement of Lt. Gen. McKenzie). The missiles all hit their
targets within a few minutes of each other, although the full operation lasted
several hours. Id.
The United States deconflicted the airspace with Russia in advance and
selected the sites to reduce the risk of hitting Russian forces. DoD Briefing
(statement of Lt. Gen. McKenzie); Mattis Briefing (statement of Gen. Dun-
ford). The strikes were timed to hit their targets around 4 a.m. local time to
reduce casualties. DoD Briefing (statement of Lt. Gen. McKenzie). The sites

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were chosen to minimize collateral damage, while inflicting damage on the
chemical-weapons program. Id. (“[T]hese are the targets that presented the
best opportunity to minimize collateral damage, to avoid killing innocent
civilians, and yet to send a very strong message.”); Mattis Briefing (state-
ment of General Dunford) (“[W]e chose these particular targets to mitigate
the risk of civilian casualties, number one. We chose these targets because
they were specifically associated with the chemical program . . . . So these
targets were carefully selected with proportionality[,] discrimination and
being specifically associated with the chemical program.”).
The allied attacks followed a limited U.S. strike in April 2017, in the
wake of Syria’s use of sarin against civilians in Khan Shaykhun. At that
time, the United States responded with fifty-eight missiles aimed at the
Shayrat airfield, which damaged or destroyed Syrian fuel and ammunition
sites, air defense capabilities, and twenty percent of the Syrian Air Force’s
operational aircraft. Remarks on United States Military Operations in Syria,
2018 Daily Comp. Pres. Doc. 201800242, at 1 (Apr. 13, 2018) (“Remarks on
Syria Operations”); Statement by Secretary of Defense Jim Mattis on the
U.S. Military Response to the Syrian Government’s Use of Chemical Weap-
ons (Apr. 10, 2017), https://www.defense.gov/News/News-Releases/News-
Release-View/Article/1146758/statement-by-secretary-of-defense-jim-
mattis-on-the-us-military-response-to-the/source/GovDelivery/. While the
April 2017 strike targeted the airfield from which the Syrian regime deliv-
ered the weapons, the 2018 attacks were focused on the long-term degrada-
tion of Syria’s capability to research, develop, and use chemical and bio-
logical weapons. Mattis Briefing (statement of Gen. Dunford).

II.

When it comes to the war powers of the President, we do not write on a
blank slate. The legal opinions of executive advisers and the still weightier
precedents of history have established that the President, as Commander in
Chief and Chief Executive, has the constitutional authority to deploy the
military to protect American persons and interests without seeking prior
authorization from Congress. See, e.g., The President and the War Power:
South Vietnam and the Cambodian Sanctuaries, 1 Op. O.L.C. Supp. 321,
331 (May 22, 1970) (“Cambodian Sanctuaries”); Training of British Flying
Students in the United States, 40 Op. Att’y Gen. 58, 62 (1941) (Jackson,
A.G.) (“British Flying Students”). The President’s authority in this area has
been elucidated by dozens of occasions over the course of 230 years, quite

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literally running from the halls of Montezuma to the shores of Tripoli and
beyond. 1 Many of those actions were approved by opinions of this Office or
of the Attorney General, and many involved engagements considerably
broader than the April 2018 Syrian strikes. The Constitution reserves to
Congress the authority to “declare War” and thereby to decide whether to
commit the Nation to a sustained, full-scale conflict with another Nation.
Yet Presidents have repeatedly engaged in more limited hostilities to ad-
vance the Nation’s interests without first seeking congressional authoriza-
tion.
The President’s authority to direct U.S. military forces arises from Article
II of the Constitution, which makes the President the “Commander in Chief
of the Army and Navy of the United States,” U.S. Const. art. II, § 2, cl. 1,
and vests in him the Executive Power, id. art. II, § 1, cl. 1. These powers
allow him “to direct the movements of the naval and military forces placed
by law at his command.” Fleming v. Page, 50 U.S. (9 How.) 603, 615
(1850). Chief Justice Marshall suggested that the President’s “high duty” to
“‘take care that the laws be faithfully executed,’” as well as his power as

1 After receiving an ultimatum from the Bey of Tripoli in May 1801, President Jefferson

dispatched U.S. ships to the Mediterranean with orders, in the event the Barbary Powers
declared war, to “distribute your force . . . so as best to protect our commerce & chastise their
insolence—by sinking, burning or destroying their ships & Vessels wherever you shall find
them.” David P. Currie, The Constitution in Congress: The Jeffersonians, 1801–1829, at 127–
28 (2001). After Tripoli declared war, the United States launched a surprise attack on a
Tripolitan vessel. Id. at 128. In reporting the action to Congress, Jefferson elided the offen-
sive nature of the attack and sought authorization to “go beyond the line of defense,” id. at
124, 128, which Congress granted on February 6, 1802. See Act of Feb. 6, 1802, ch. IV, § 2,
2 Stat. 129, 130.
After Congress annexed Texas, President Polk deployed the U.S. military 150 miles
south of the disputed border with Mexico to the Rio Grande in June 1845. See David P.
Currie, The Constitution in Congress: Descent into the Maelstrom, 1829–1861, at 102
(2005); 4 A Compilation of the Messages and Papers of the Presidents, 1788–1897, at 437,
440 (James D. Richardson ed., 1897); see also Cambodian Sanctuaries, 1 Op. O.L.C. Supp.
at 327. After active hostilities commenced, Congress declared war. See Act of May 13,
1846, ch. XVI, 9 Stat. 9 (1846); see also The Prize Cases, 67 U.S. (2 Black) 635, 668
(1863) (“The battles of Palo Alto and Resaca de la Palma had been fought before the
passage of the Act of Congress of May 13th, 1846, which recognized ‘a state of war as
existing by the act of the Republic of Mexico.’ This act not only provided for the future
prosecution of the war, but was itself a vindication and ratification of the Act of the
President in accepting the challenge without a previous formal declaration of war by
Congress.”).

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Commander in Chief, imply some authority to deploy U.S. military force.
Little v. Barreme, 6 U.S. (2 Cranch) 170, 177 (1804). The Supreme Court
has recognized that the President holds the “vast share of responsibility for
the conduct of our foreign relations,” Am. Ins. Ass’n v. Garamendi, 539 U.S.
396, 414 (2003) (internal quotation marks omitted), and holds “independent
authority in the areas of foreign policy and national security,” id. at 429
(internal quotation marks omitted); see also Dep’t of Navy v. Egan, 484 U.S.
518, 529 (1988) (“The Court also has recognized the generally accepted
view that foreign policy [is] the province and responsibility of the Execu-
tive.”) (internal quotation marks omitted). By its terms, Article II provides
the President with the authority to direct U.S. military forces in engagements
necessary to advance American national interests abroad.
In evaluating the division of authority between the President and Con-
gress, the Supreme Court has placed “significant weight” on “accepted
understandings and practice.” Zivotofsky v. Kerry, 135 S. Ct. 2076, 2091
(2015); see NLRB v. Noel Canning, 134 S. Ct. 2550, 2559 (2014) (noting
that “long settled and established practice is a consideration of great weight
in a proper interpretation of constitutional provisions regulating the relation-
ship between Congress and the President” (internal quotation marks and
alterations omitted)); Dames & Moore v. Regan, 453 U.S. 654, 678–86
(1981) (describing “a history of congressional acquiescence in conduct of
the sort engaged in by the President”). We have recognized that “[s]ince
judicial precedents are virtually non-existent” in defining the scope of the
President’s war powers, “the question is one which of necessity must be
decided by historical practice.” Presidential Authority to Permit Incursion
Into Communist Sanctuaries in the Cambodia-Vietnam Border Area, 1 Op.
O.L.C. Supp. 313, 317 (May 14, 1970) (“Vietnam Border Area”).
And that history points strongly in one direction. While our Nation has
sometimes debated the scope of the President’s war powers under the Con-
stitution, his authority to direct U.S. forces in hostilities without prior con-
gressional authorization is supported by a “long continued practice on the
part of the Executive, acquiesced in by the Congress.” Cambodian Sanctuar-
ies, 1 Op. O.L.C. Supp. at 326; see also Deployment of United States Armed
Forces to Haiti, 28 Op. O.L.C. 30, 31 (2004) (“Haiti Deployment II ”) (“His-
tory offers ample evidence for the proposition that the President may take
military action abroad, even, as here, in the absence of specific prior con-
gressional authorization.”); Presidential Power to Use the Armed Forces
Abroad Without Statutory Authorization, 4A Op. O.L.C. 185, 187 (1980)

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(“Presidential Power”) (“Our history is replete with instances of presidential
uses of military force abroad in the absence of prior congressional approv-
al.”).
Presidents have exercised their authority to direct military operations
without congressional authorization since the earliest days of the Republic.
President Washington directed offensive operations against the Wabash
Indians in 1790. See David P. Currie, The Constitution in Congress: The
Federalist Period, 1789–1801, at 84 (1997) (“[B]oth Secretary [of War]
Knox and [President] Washington himself seemed to think [the Commander
in Chief ] authority extended to offensive operations undertaken in retalia-
tion for Indian atrocities.”). As noted above, the Jefferson Administration
instructed the United States Navy to “sink[], burn[] or destroy[]” Barbary
cruisers. See supra note 1; see also Authority to Use United States Military
Forces in Somalia, 16 Op. O.L.C. 6, 9 (1992) (“Somalia Deployment ”).
These past deployments have included President Truman’s defense of South
Korea; President Kennedy’s introduction of U.S. forces into Vietnam;
President Reagan’s retaliatory strikes on Libya following the Beirut bomb-
ing; President George H.W. Bush’s introduction of U.S. troops into Somalia;
President Clinton’s actions in Bosnia, Haiti, Kosovo, Sudan, and Afghani-
stan; President George W. Bush’s intervention in Haiti; and President
Obama’s airstrikes in Libya and in Houthi-controlled territory in Yemen.
While the precise counting varies, by the middle of the twentieth century,
scholars had identified well over 100 instances of military deployments
without prior congressional authorization. See Proposed Deployment of
United States Armed Forces into Bosnia, 19 Op. O.L.C. 327, 331 (1995)
(“Bosnia Deployment ”) (“In at least 125 instances, the President acted
without express authorization from Congress.”); William Gabriel Carras,
The Analysis and Interpretation of the Use of Presidential Authority to
Order United States Armed Forces into Military Action in Foreign Territo-
ries Without a Formal Declaration of War 369 (1959) (identifying 124 of
141 military deployments between 1798 and 1956); James Grafton Rogers,
World Policing and the Constitution 93–123 (1945) (identifying 119 of 149
military deployments between 1798 and 1941). In the forty-five years since
the 1973 enactment of the War Powers Resolution, Pub. L. No. 93-148, 87
Stat. 555, Presidents have submitted more than eighty reports of hostilities to
Congress that did not rely upon statutory authorization. See Matthew C.
Weed, Cong. Research Serv., R42699, The War Powers Resolution: Con-
cepts and Practice 57–83 (Mar. 28, 2017). From the border of the Rio

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Grande to the thirty-eighth parallel on the Korean peninsula, from the Gulf
of Tonkin to the Shayrat Airfield, Presidents have acted, and Congress has
accepted or ratified the President’s use of the military, to advance our na-
tional interests.
As Assistant Attorney General Rehnquist observed, “[i]t is too plain” in
view of this record “to admit of denial that the Executive, under his power as
Commander in Chief, is authorized to commit American forces in such a
way as to seriously risk hostilities, and also to actually commit them to such
hostilities, without prior congressional approval.” Cambodian Sanctuaries,
1 Op. O.L.C. Supp. at 331. That historical record has only expanded in the
decades since Vietnam. Since then, in light of “repeated past practice under
many Presidents,” this Office has repeatedly advised that “the President has
the power to commit United States troops abroad for the purpose of protect-
ing important national interests.” Somalia Deployment, 16 Op. O.L.C. at 9;
see also Authority to Use Military Force in Libya, 35 Op. O.L.C. ___, at *6
(Apr. 1, 2011) (“Libya Deployment ”), https://www.justice.gov/sites/default/
files/olc/opinions/2011/04/31/authority-military-use-in-libya_0.pdf; Haiti
Deployment II, 28 Op. O.L.C. at 31. Congress likewise acknowledged this
authority in the War Powers Resolution, at least implicitly, by recognizing
that the President may introduce U.S. forces into hostilities for up to sixty
days or more without congressional authorization. 50 U.S.C. § 1544(b); see
also Deployment of United States Armed Forces into Haiti, 18 Op. O.L.C.
173, 176 (1994) (“Haiti Deployment I ”). 2
Although “[t]he limits of the President’s power as Commander in Chief
are nowhere defined in the Constitution,” we have recognized a “negative

2 The War Powers Resolution does not constitute an affirmative source of authority for the
President to introduce U.S. forces into hostilities, 50 U.S.C. § 1547(d)(2), but it also is not
“intended to alter the constitutional authority . . . of the President.” Id. § 1547(d)(1). By
seeking to require the cessation of hostilities within sixty days, absent congressional authori-
zation, the statute assumes that the President has the authority to authorize such engagements.
The statute begins with a statement of purpose and policy that identifies a narrow set of
engagements that the President may direct without congressional authorization. Id. § 1541(c).
Yet we have recognized that this policy statement neither affirmatively limits presidential
authority nor constitutes an exhaustive list of the circumstances in which the President may
use military force to protect important national interests. See, e.g., Overview of the War
Powers Resolution, 8 Op. O.L.C. 271, 274 (1984); see also Authority of the President Under
Domestic and International Law to Use Military Force Against Iraq, 26 Op. O.L.C. 143,
159–61 (2002) (summarizing the Executive Branch’s longstanding constitutional concerns
with the War Powers Resolution).

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implication from the fact that the power to declare war is committed to
Congress.” Cambodian Sanctuaries, 1 Op. O.L.C. Supp. at 325. The Con-
stitution reserves to Congress the power to “declare War,” U.S. Const.
art. I, § 8, cl. 11, and the authority to fund military operations, id. art. I,
§ 8, cl. 12. This was a deliberate choice of the Founders, who sought to
prevent the President from bringing the Nation into a full-scale war with-
out the authorization of Congress. See, e.g., The Federalist No. 69, at 465
(Alexander Hamilton) (Jacob E. Cooke ed., 1961) (recognizing that the
President lacks the authority of the British King, which “extends to the
declaring of war and to the raising and regulating of fleets and armies; all
which by the Constitution under consideration would appertain to the
Legislature”); 4 Jonathan Elliot, The Debates in the Several State Conven-
tions of the Adoption of the Federal Constitution 107–08 (2d ed. 1836)
(James Iredell, speaking at the North Carolina Ratifying Convention)
(“The President has not the power of declaring war by his own authority,
nor that of raising fleets and armies. These powers are vested in other
hands. The power of declaring war is expressly given to Congress, that is,
to the two branches of the legislature . . . . They have also expressly dele-
gated to them the powers of raising and supporting armies, and of provid-
ing and maintaining a navy.”); United States v. Smith, 27 F. Cas. 1192,
1230 (C.C.D.N.Y. 1806) (No. 16,342) (“[I]t is the exclusive province of
congress to change a state of peace into a state of war.”). These legislative
powers ensure that the use of force “cannot be sustained over time without
the acquiescence, indeed the approval, of Congress, for it is Congress that
must appropriate the money to fight a war or a police action.” Presidential
Power, 4A Op. O.L.C. at 188. These powers further oblige the President to
seek congressional approval prior to contemplating military action that
would bring the Nation into a war.
Not every military operation, however, rises to the level of a war. Rather,
“the historical practice of military action without congressional approval
precludes any suggestion that Congress’s authority to declare war covers
every military engagement, however limited, that the President initiates.”
Libya Deployment at *8. Early on, the Supreme Court distinguished between
a declared war (which arises where “one whole nation is at war with another
whole nation” with hostilities arising “in every place, and under every
circumstance”) and a more limited engagement, an “imperfect war” (in
which hostilities are “more confined in its nature and extent; being limited as
to places, person and things”). Bas v. Tingy, 4 U.S. (4 Dall.) 37, 40–41

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(1800). 3 Consistent with that early recognition, we have repeatedly distin-
guished between limited hostilities and “prolonged and substantial military
engagements, typically involving exposure of U.S. military personnel to
significant risk over a substantial period.” Libya Deployment at *8.
When reviewing proposed military engagements, this Office has recog-
nized that “a planned military engagement that constitutes a ‘war’ within the
meaning of the Declaration of War Clause may require prior congressional
authorization.” Id.; see also Cambodian Sanctuaries, 1 Op. O.L.C. Supp. at
331–32 (“[I]f the contours of the divided war power contemplated by the
framers of the Constitution are to remain, constitutional practice must in-
clude executive resort to Congress in order to obtain its sanction for the
conduct of hostilities which reach a certain scale.”); Vietnam Border Area,
1 Op. O.L.C. Supp. at 317 (“Under our Constitution it is clear that Congress
has the sole authority to declare formal, all-out war.”). We have therefore
considered the scale of the expected hostilities in analyzing whether a pro-
posed engagement would constitute a war for constitutional purposes. See
Libya Deployment at *8–9; Haiti Deployment I, 18 Op. O.L.C. at 177–78.

III.

We now explain our analysis of the April 13, 2018 Syrian strikes in light
of our precedents. In evaluating whether a proposed military action falls
within the President’s authority under Article II of the Constitution, we have
distilled our precedents into two inquiries. First, we consider whether the
President could reasonably determine that the action serves important na-
tional interests. See, e.g., Somalia Deployment, 16 O.L.C. at 9 (“At the core
of this power is the President’s authority to take military action to protect

3 Bas concerned the Quasi-War with France, which involved hostilities that Congress had

authorized by statute without a formal declaration of war. See Treason, 1 Op. Att’y Gen. 84,
84 (1798) (“Having taken into consideration the acts of the French republic relative to the
United States, and the laws of Congress passed at the last session, it is my opinion that there
exists not only an actual maritime war between France and the United States, but a maritime
war authorized by both nations.”). We do not suggest that every “imperfect war” falls within
the sphere of unilateral executive action. As with the Quasi-War, Congress may authorize the
use of force in such conflicts, and we do not rule out that some imperfect wars may involve
such prolonged and substantial engagements as to require that authorization. Our point though
is that the early Supreme Court recognized the distinction between wars that must be declared
under Article I, Section 8 of the Constitution and more limited military engagements—many
of which have not traditionally been authorized by Congress.

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American citizens, property, and interests from foreign threats.”); British
Flying Students, 40 Op. Att’y Gen. at 62 (“[T]he President’s authority has
long been recognized as extending to the dispatch of armed forces outside of
the United States, either on missions of good will or rescue, or for purposes
of protecting American lives or property or American interests.”). Second,
we consider whether the “anticipated nature, scope and duration” of the
conflict might rise to the level of a war under the Constitution. See Libya
Deployment at *9 (quoting Haiti Deployment I, 18 Op. O.L.C. at 179). Prior
to the Syrian strikes, we applied this framework to conclude that the pro-
posed Syrian operation would fall within the President’s constitutional
authority.

A.

This Office has recognized that a broad set of interests would justify use
of the President’s Article II authority to direct military force. These interests
understandably grant the President a great deal of discretion. The scope of
U.S. involvement in the world, the presence of U.S. citizens across the
globe, and U.S. leadership in times of conflict, crisis, and strife require that
the President have wide latitude to protect American interests by responding
to regional conflagrations and humanitarian catastrophes as he believes
appropriate. The Commander in Chief bears great responsibility for the use
of the armed forces and for putting U.S. forces in harm’s way. We would not
expect that any President would use this power without a substantial basis
for believing that a proposed operation is necessary to advance important
interests of the Nation. The aim of this inquiry is not to evaluate the worth of
the interests at stake—a question more of policy than of law—but rather, to
set forth the justifications for the President’s use of military force and to
situate those interests within a framework of prior precedents.
In our past opinions, this Office has identified a number of different inter-
ests that have supported sending U.S. forces into harm’s way, including the
following:
• the protection of U.S. persons and property, see, e.g., Presidential
Power, 4A Op. O.L.C. at 187 (“Presidents have repeatedly employed
troops abroad in defense of American lives and property.”); Haiti De-
ployment II, 28 Op. O.L.C. at 31 (“The President has the authority to
deploy the armed forces abroad in order to protect American citizens
and interests from foreign threats.”);

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• assistance to allies, see, e.g., Haiti Deployment I, 18 Op. O.L.C. at 79
(approving of intervention “at the invitation of a fully legitimate gov-
ernment”); Presidential Power, 4A Op. O.L.C. at 187–88 (citing the
Korean War as “precedent . . . for the commitment of United States
armed forces, without prior congressional approval or declaration of
war, to aid an ally in repelling an armed invasion”);
• support for the United Nations, see, e.g., Haiti Deployment II, 28 Op.
O.L.C. at 33 (“Another American interest in Haiti arises from the in-
volvement of the United Nations in the situation there.”); Somalia De-
ployment, 16 Op. O.L.C. at 11 (“[M]aintaining the credibility of United
Nations Security Council decisions, protecting the security of United
Nations and related relief efforts, and ensuring the effectiveness of
United Nations peacekeeping operations can be considered a vital na-
tional interest[.]”); and
• promoting regional stability, see, e.g., Haiti Deployment II, 29 Op.
O.L.C. at 32 (“The President also may determine that the deployment
is necessary to protect American foreign policy interests. One such in-
terest is the preservation of regional stability.”); Libya Deployment at
*12 (“[W]e believe the President could reasonably find a significant
national security interest in preventing Libyan instability from spread-
ing elsewhere in this critical region.”).
In recent years, we have also identified the U.S. interest in mitigating hu-
manitarian disasters. See Memorandum Opinion for the Counsel to the
President, from Karl R. Thompson, Principal Deputy Assistant Attorney
General, Office of Legal Counsel, Re: Authority to Use Military Force in
Iraq at 20–24 (Dec. 30, 2014) (“Iraq Deployment ”). With respect to Syria,
in April 2017, the President identified the U.S. interest in preventing the use
and proliferation of chemical weapons, see Letter to Congressional Leaders
on United States Military Operations in Syria, 2017 Daily Comp. Pres. Doc.
201700244, at 1 (Apr. 8, 2017) (“2017 Congressional Notification”). As
explained below, these interests too are consistent with those that the Presi-
dent and his advisers have relied upon in the past.
The President identified three interests in support of the April 2018 Syria
strikes: the promotion of regional stability, the prevention of a worsening of
the region’s humanitarian catastrophe, and the deterrence of the use and
proliferation of chemical weapons. See Letter to Congressional Leaders on
United States Military Operations in Syria, 2018 Daily Comp. Pres. Doc.
201800243, at 1 (Apr. 15, 2018). Prior to the attack, we advised that the

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President could reasonably rely on these national interests to authorize air
strikes against particular facilities associated with Syria’s chemical-weapons
program without congressional authorization.
As discussed above, Presidents have deployed U.S. troops on multiple
occasions in the interest of promoting regional stability and preventing the
spread of an ongoing conflict. While the United States is not the world’s
policeman, as its power has grown, the breadth of its regional interests has
expanded and threats to national interests posed by foreign disorder have
increased. See, e.g., Authority of the President to Repel the Attack in Korea,
23 Dep’t of State Bull. 173, 175 (1950) (“Attack in Korea”) (quoting Secre-
tary of State Hay’s statement that President McKinley dispatched troops to
China during the Boxer rebellion in part to “prevent a spread of the disor-
ders”); Clarence W. Berdahl, War Powers of the Executive of the United
States 53–55 (1921) (describing numerous instances of the deployment of
troops to secure stability in the Caribbean). This Office has consistently
recognized that U.S. national interests in regional stability may support
military intervention. See Haiti Deployment II, 28 Op. O.L.C. at 32 (“The
President also may determine that the deployment is necessary to protect
American foreign policy interests. One such interest is the preservation of
regional stability.”); Bosnia Deployment, 19 Op. O.L.C. at 332–33 (“[Mili-
tary deployment] would serve significant national security interests, by
preserving peace in the region and forestalling the threat of a wider con-
flict.”); Libya Deployment at *10 (concluding the combination of interests in
“preserving regional stability and supporting the [United Nation Security
Council’s] credibility and effectiveness” were a “sufficient basis for the
President’s exercise of his constitutional authority to order the use of mili-
tary force”).
Here, the President could reasonably determine that Syria’s use of chemi-
cal weapons in the ongoing civil war threatens to undermine further peace
and security of the Near East, a region that remains critically important to
our national security. Syria’s possession and use of chemical weapons have
increased the risk that others will gain access to them. See Daniel R. Coats,
Director of National Intelligence, Statement for the Record: Worldwide
Threat Assessment of the US Intelligence Community at 7 (Feb. 13, 2018),
https://www.dni.gov/files/documents/Newsroom/Testimonies/2018-ATA---
Unclassified-SSCI.pdf (“Worldwide Threat Assessment”) (“Biological and
chemical materials and technologies—almost always dual-use—move easily
in the globalized economy, as do personnel with the scientific expertise to

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 April 2018 Airstrikes Against Syrian Chemical-Weapons Facilities

design and use them for legitimate and illegitimate purposes.”). The prolif-
eration of such weapons to other countries with fragile governments or to
terrorist groups could further spread conflict and disorder within the region.
See Council on Foreign Relations, A Conversation with Nikki Haley (Mar.
29, 2017), https://www.cfr.org/event/conversation-nikki-haley (“Let’s really
look at the fact that if we don’t have a stable Syria, we don’t have a stable
region.”); Remarks to the United Nations General Assembly in New York
City, 2017 Daily Comp. Pres. Doc. 201700658, at 5 (Sept. 19, 2017), (“No
society can be safe if banned chemical weapons are allowed to spread.”);
United States Mission to the United Nations, Ambassador Haley Delivers
Remarks at a UN Security Council Meeting on Nonproliferation (Jan. 18,
2018), https://usun.state.gov/highlights/8276 (“The regimes that most threat-
en the world today with weapons of mass destruction are also the source of
different kinds of security challenges. They deny human rights and funda-
mental freedoms to their people. They promote regional instability. They aid
terrorists and militant groups. They promote conflict that eventually spills
over its borders.”). The United States has a direct interest in ensuring that
others in the region not look to Syria’s use of chemical weapons as a suc-
cessful precedent for twenty-first-century conflicts.
Moreover, the regime’s use of chemical weapons is a particularly egre-
gious part of a broader destabilizing conflict. The civil war in Syria directly
empowered the growth of the Islamic State of Iraq and Syria (“ISIS”), a
terrorist threat that has required the deployment of over 2,000 U.S. troops.
See Jim Garmone, DoD News, Defense Media Activity, Pentagon Announc-
es Troop Levels in Iraq, Syria (Dec. 6, 2017), https://www.defense.gov/
News/Article/Article/1390079/pentagon-announces-troop-levels-in-iraq-
syria/. 4 The instability in Syria has had a direct and marked impact upon the
national security of close American allies and partners, including Iraq,
Israel, Jordan, Lebanon, and Turkey, all of which border Syria and have had
to deal with unrest from the conflict. Rand Corporation, Research Brief, The
Conflict in Syria: Understanding and Avoiding Regional Spillover Effects
at 1 (2014), https://www.rand.org/content/dam/rand/pubs/research_briefs/
RB9700/RB9785/RAND_RB9785.pdf; see also generally Leïla Vignal, The

4 The U.S. deployment against ISIS is supported by congressional authorization pursuant

to the 2001 Authorization for Use of Military Force, Pub. L. No. 107-40, 115 Stat. 224, and
the Authorization for Use of Military Force Against Iraq Resolution of 2002, Pub. L. No.
107-243, 116 Stat. 1498. U.S. actions to counter ISIS in Syria are therefore based upon a
different legal footing than are the attacks against Syria’s chemical-weapons facilities.

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Changing Borders and Borderlands of Syria in a Time of Conflict, 93 Int’l
Affairs 809 (2017). In addition, the power vacuum in Syria has provided an
opportunity for Russia and Iran to deepen their presence in the region and
engage in activities that have had a directly adverse impact on the interests and
security of the United States and its allies in the area. See President Donald J.
Trump, National Security Strategy of the United States of America at 49 (Dec.
2017), https://www.whitehouse.gov/wp-content/uploads/2017/12/NSS-Final-
12-18-2017-0905.pdf (“Rival states are filling vacuums created by state
collapse and prolonged regional conflict.”).
The Syrian regime’s continued attacks on civilians have also contributed to
the displacement of civilians and thus deepened the instability in the region.
According to the Director of National Intelligence, as of October 2017, more
than 5 million Syrian refugees had fled to neighboring countries and more
than 6 million were displaced internally. See Worldwide Threat Assessment at
21; see also Arwa Damon and Gul Tuysuz, CNN, Survivors of a Chemical
Attack in Syria Tell Their Stories for the First Time (Apr. 16, 2018), https://
www.cnn.com/2018/04/15/middleeast/douma-chemical-attack-survivors-
stories-arwa-damon-intl/index.html (interviewing individuals at a refugee
camp who survived the chemical-weapons attack on Douma). These large-
scale population movements have added to unrest throughout the region. Cf.
Libya Deployment at *11 (explaining that the flight of civilians to neighboring
countries was “destabilizing the peace and security of the region” (internal
quotation marks omitted)).
In directing the strikes, the President also relied on the national interest in
mitigating a humanitarian crisis. In analyzing proposed military operations
in Iraq designed to prevent genocidal acts against the Yazidis and otherwise
to protect civilians at risk, we advised that humanitarian concerns could
provide a basis for the President’s use of force under his constitutional
authority. See Iraq Deployment at 20–24. Given the role of the United States
in the international community and the humanitarian interests of its people,
Presidents have on many occasions deployed troops to prevent or mitigate
humanitarian disasters. See, e.g., Letter to Congressional Leaders on De-
ployment of United States Armed Forces to Haiti (Sept. 18, 1994), 2 Pub.
Papers of Pres. William J. Clinton 1572, 1572 (1994) (“The deployment of
U.S. Armed Forces into Haiti is justified by United States national security
interests” including “stop[ping] the brutal atrocities that threaten tens of
thousands of Haitians”); Letter to Congressional Leaders Reporting on the
Commencement of Military Operations Against Libya (Mar. 21, 2011),

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 April 2018 Airstrikes Against Syrian Chemical-Weapons Facilities

1 Pub. Papers of Pres. Barack Obama 280, 280 (2011) (notifying Congress
of the commencement of operations “to prevent a humanitarian catastro-
phe”).
In some cases, humanitarian concerns have been a significant, or even the
primary, interest served by U.S. military operations. In 1992, when President
George H.W. Bush announced that he had ordered the deployment of “a
substantial American force” to Somalia during a widespread famine, he
described it as “a mission that can ease suffering and save lives.” Address to
the Nation on the Situation in Somalia (Dec. 4, 1992), 2 Pub. Papers of
Pres. George Bush 2174, 2174–75 (1992–93); see also id. at 2175 (“Let me
be very clear: Our mission is humanitarian[.]”); Somalia Deployment, 16
Op. O.L.C. at 6 (“I am informed that the mission of those troops will be to
restore the flow of humanitarian relief to those areas of Somalia most affect-
ed by famine and disease[.]”). Similarly, military intervention in Bosnia
included the establishment of a no-fly zone, maintained for roughly two-
and-a-half years, in support of a humanitarian air drop. Daniel L. Haulman,
The United States Air Force and Bosnia, 1992–1995, Air Power History 24,
35 (2013); Letter to Congressional Leaders Reporting on the No-Fly Zone in
Bosnia-Herzegovina (Oct. 13, 1993), 2 Pub. Papers of Pres. William J. Clin-
ton 1740, 1741 (1993) (“[T]he no-fly zone enforcement operations have
been militarily effective and have reduced potential air threats to our human-
itarian airlift and airdrop flights.”); Address Before a Joint Session of the
Congress on the State of the Union (Jan. 25, 1994), 1 Pub. Papers of Pres.
William J. Clinton 126, 132 (1994) (noting the continuation of the “longest
humanitarian air lift in history in Bosnia”); Address to the Nation on Imple-
mentation of the Peace Agreement in Bosnia-Herzegovina (Nov. 27, 2995),
2 Pub. Papers of Pres. William J. Clinton 1784, 1785 (1995) (“We used our
airpower to conduct the longest humanitarian airlift in history and to enforce
a no-fly zone that took the war out of the skies.”) (“Clinton Address to the
Nation”). President Clinton also framed U.S. peacekeeping efforts in human-
itarian terms. Clinton Address to the Nation at 1784 (“In fulfilling this
mission, we will have the chance to help stop the killing of innocent civil-
ians, especially children[.]”).
The Syrian regime’s use of chemical weapons has contributed to the on-
going humanitarian crisis in Syria. As discussed above, civilians fleeing
from the strikes become refugees needing assistance. See Carla E. Humud et
al., Cong. Research Serv., RL33487, Armed Conflict in Syria: Overview and
U.S. Response 19 (Apr. 18, 2018) (explaining that 13.1 million people in

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Syria were in need of humanitarian assistance as of early 2018, more than
two-thirds of the country’s 18 million people). Internally displaced persons
in Syria often lack access to basic services or medical care, see World
Health Organization, Syrian Arab Republic Humanitarian Response Plan
2018, http://www.who.int/emergencies/response-plans/2018/syria/en/ (last
visited May 30, 2018), difficulties that are heightened for victims of chemi-
cal-weapons attacks. But even where the attacks do not displace civilians,
the nature of chemical weapons alone makes their use a humanitarian issue.
See Remarks on Syria Operations at 1 (“The evil and the despicable attack
left mothers and fathers, infants and children, thrashing in pain and gasping
for air. These are not the actions of a man; they are crimes of a monster
instead.”). As the President explained after the Syrian strike, “[c]hemical
weapons are uniquely dangerous not only because they inflict gruesome
suffering, but because even small amounts can unleash widespread devasta-
tion.” Id.
In carrying out these strikes, the President also relied on the national in-
terest in deterring the use and proliferation of chemical weapons. The Presi-
dent previously relied upon this interest in ordering the April 2017 airstrike
in response to the attack on Khan Shaykhun. See 2017 Congressional Notifi-
cation (stating that the President directed a strike on the Shayrat military
airfield to “degrade the Syrian military’s ability to conduct further chemical
weapons attacks and to dissuade the Syrian regime from using or proliferat-
ing chemical weapons, thereby promoting the stability of the region and
averting a worsening of the region’s current humanitarian catastrophe”).
While we are unaware of prior Presidents justifying U.S. military actions
based on this interest as a matter of domestic law, we believe that it is con-
sistent with those that have justified previous uses of force. The United
States has long and consistently objected to the use and proliferation of
chemical weapons. See Protocol for the Prohibition of the Use in War of
Asphyxiating, Poisonous, or Other Gases, and of Bacteriological Methods of
Warfare, adopted June 17, 1925, 26 U.S.T. 571, 94 L.N.T.S. 65; The Axis Is
Warned Against the Use of Poison Gas (June 8, 1948), Pub. Papers of Pres.
Franklin D. Roosevelt 242, 243 (1943) (“Use of [chemical] weapons has
been outlawed by the general opinion of civilized mankind.”). For nearly
thirty years, Presidents have repeatedly declared the proliferation of chemi-
cal weapons to be a national emergency. See Notice Regarding the Contin-
uation of the National Emergency with Respect to the Proliferation of
Weapons of Mass Destruction, 82 Fed. Reg. 51,971 (Nov. 6, 2017) (most

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 April 2018 Airstrikes Against Syrian Chemical-Weapons Facilities

recent order continuing in effect an emergency first declared in Executive
Order 12735 of Nov. 16, 1990). In 1997, the United States ratified the
Chemical Weapons Convention, which prohibits the use, development,
production, and retention of chemical weapons. See Remarks on Senate
Ratification of the Chemical Weapons Convention and an Exchange with
Reporters (Apr. 24, 1997), 1 Pub. Papers of Pres. William J. Clinton 480,
480 (1997) (stating that ratification will permit the end of “a century that
began with the horror of chemical weapons in World War I much closer to
the elimination of those kinds of weapons”). And Congress cited Iraq’s
development of chemical weapons as one of the reasons in support of au-
thorizing the use of military force against Iraq in 2002. See Authorization for
Use of Military Force Against Iraq Resolution of 2002, Pub. L. No. 107-243,
116 Stat. 1498, 1498 (“Whereas the efforts of international weapons inspec-
tors, United States intelligence agencies, and Iraqi defectors led to the dis-
covery that Iraq had large stockpiles of chemical weapons”).
The United States has also repeatedly joined international condemnation
of Syria’s use of chemical weapons. See, e.g., S.C. Res. 2319 (Nov. 17,
2016) (“Condemning again in the strongest terms any use of any toxic
chemicals as a weapon in the Syrian Arab Republic and expressing alarm
that civilians continue to be killed and injured by toxic chemicals as weap-
ons in the Syrian Arab Republic”); S.C. Res. 2235 (Aug. 7, 2015) (“Con-
demning in the strongest terms any use of any toxic chemical as a weapon in
the Syrian Arab Republic and noting with outrage that civilians continue to
be killed and injured by toxic chemicals as weapons in the Syrian Arab
Republic, Reaffirming that the use of chemical weapons constitutes a serious
violation of international law, and stressing again that those individuals
responsible for any use of chemical weapons must be held accountable”);
S.C. Res. 2209 (Mar. 6, 2015) (“Reaffirming that the use of chemical weap-
ons constitutes a serious violation of international law and reiterating that
those individuals responsible for any use of chemical weapons must be held
accountable”); S.C. Res. 2118 (Sept. 2017, 2013) (“Determining that the use
of chemical weapons in the Syrian Arab Republic constitutes a threat to
international peace and security”).
Despite near-global condemnation, a small number of state and non-state
actors persist in using chemical weapons, and Syria’s continued use of them
“threatens to desensitize the world to their use and proliferation, weaken
prohibitions against their use, and increase the likelihood that additional
states will acquire and use these weapons.” USG Assessment. Last year’s

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U.S. strike did not fully dissuade the Syrian regime from continuing to use
chemical weapons. And Russia recently used a nerve agent in an attempted
assassination in the United Kingdom, “showing an uncommonly brazen
disregard for the taboo against chemical weapons.” Id.; see also United
States Mission to the United Nations, Ambassador Haley Delivers Remarks
at a UN Security Council Briefing on Chemical Weapons Use in Syria
(Apr. 4, 2018), https://usun.state.gov/highlights/8366 (“When we let one
regime off the hook, others take notice. The use of nerve agents in Salisbury
and Kuala Lumpur proves this point and reveals a dangerous trend. We are
rapidly sliding backward, crossing back into a world that we thought we
left.”). ISIS has also acquired and deployed chemical weapons. See World-
wide Threat Assessment at 8. The United States has a weighty interest in
deterring the use of these weapons.
In sum, the President here was faced with a grave risk to regional stabil-
ity, a serious and growing humanitarian disaster, and the use of weapons
repeatedly condemned by the United States and other members of the inter-
national community. In such circumstances, the President could reasonably
conclude that these interests provided a basis for airstrikes on facilities that
support the regime’s use of chemical weapons. See Attack in Korea, 23
Dep’t of State Bull. at 174 (“The United States has, throughout its history,
upon orders of the Commander in Chief to the Armed Forces and without
congressional authorization, acted to prevent violent and unlawful acts in
other states from depriving the United States and its nationals of the benefits
of such peace and security.”). We believe that these interests fall comforta-
bly within those that our Office has previously relied upon in concluding
that the President had appropriately exercised his authority under Article II,
and we so advised prior to the Syrian strikes.

B.

We next considered whether the President could expect the Syrian opera-
tions to rise to the level of a war requiring congressional authorization. Such
a determination “requires a fact-specific assessment of the ‘anticipated
nature, scope, and duration’ of the planned military operations.” Libya
Deployment at *8 (quoting Haiti Deployment I, 18 Op. O.L.C. at 179). As
we have previously explained, military operations will likely rise to the level
of a war only when characterized by “prolonged and substantial military
engagements, typically involving exposure of U.S. military personnel to
significant risk over a substantial period.” Id.

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 April 2018 Airstrikes Against Syrian Chemical-Weapons Facilities

We have found that previous military deployments did not rise to the level
of a war even where the deployment was substantial. For example, the
United States spent two years enforcing a no-fly zone, protecting United
Nations (“UN”) peacekeeping forces, and securing safe areas for civilians in
Bosnia, all without congressional authorization. See Bosnia Deployment, 19
Op. O.L.C. at 329 & n.2 (noting the plan to deploy 20,000 ground troops to
Bosnia as well as additional troops to surrounding areas in a support capaci-
ty); see also Libya Deployment at *9; (noting “one two-week operation in
which NATO attacked hundreds of targets and the United States alone flew
over 2300 sorties”). Similarly, in 1994, we approved a plan to deploy as
many as 20,000 troops to Haiti. Haiti Deployment I, 18 Op. O.L.C. at 179
n.10. We also approved a U.S.-led air campaign in Libya in 2011 that lasted
for over a week and involved the use of over 600 missiles and precision-
guided munitions. See DoD News Briefing with Vice Adm. Gortney from
the Pentagon on Libya Operation Odyssey Dawn (Mar. 28, 2011), http://
archive.defense.gov/transcripts/transcript.aspx?transcriptid=4803. In none of
these cases did we conclude that prior congressional authorization was
necessary.
In reviewing these deployments, we considered whether U.S. forces were
likely to encounter significant armed resistance and whether they were likely
to “suffer or inflict substantial casualties as a result of the deployment.”
Haiti Deployment I, 18 Op. O.L.C. at 179. In this regard, we have looked
closely at whether an operation will require the introduction of U.S. forces
directly into the hostilities, particularly with respect to the deployment of
ground troops. The deployment of ground troops “is an essentially different,
and more problematic, type of intervention,” given “the difficulties of disen-
gaging ground forces from situations of conflict, and the attendant risk that
hostilities will escalate.” Bosnia Deployment, 19 Op. O.L.C. at 333. In such
circumstances, “arguably there is a greater need for approval at the outset for
the commitment of such troops to such situations.” Id.
In connection with reviewing the proposed peacekeeping operations in
Bosnia, we noted that U.S. forces enforcing the no-fly zone had “engaged in
combat,” including the destruction of three aircraft violating the no-fly ban
and the downing of a fourth, and engaging Bosnian-Serb aircraft and gun-
ners. See Bosnia Deployment, 19 Op. O.L.C. at 328 (also noting airstrikes in
response to Serb air attacks threatening UN peacekeeping forces). We noted
that the peacekeeping force would require the deployment of 20,000 ground
troops to Bosnia, which would “raise[] the risk that the United States

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w[ould] incur (and inflict) casualties.” Id. at 333. Nonetheless, while “com-
bat conceivably may occur during the course of the operation,” we did not
believe it was “likely that the United States [would] find itself involved in
extensive or sustained hostilities.” Id. at 332–33 (emphases added). In Soma-
lia, we approved the introduction of U.S. combat-equipped forces to ensure
the protection of noncombatant forces involved in UN humanitarian relief.
See Somalia Deployment, 16 Op. O.L.C. at 10 (“It is also essential to con-
sider the safety of the troops to be dispatched as requested by Security
Council Resolution No. 794. The President may provide those troops with
sufficient military protection to insure that they are able to carry out their
humanitarian tasks safely and efficiently.”). And in approving the deploy-
ment of U.S. Marines to Haiti in 2004, we noted that it was “possible that
some level of violence and instability will continue.” Haiti Deployment II,
28 Op. O.L.C. at 34 (quoting Presidential Power, 4A Op. O.L.C. at 194);
see also Presidential Power, 4A Op. O.L.C. at 187 (“Operations of rescue
and retaliation have also been ordered by the President without congression-
al authorization even when they involved hostilities.”). Thus, even in cases
involving the deployment of ground troops, we have found that the expected
hostilities would fall short of a war requiring congressional authorization.
With these precedents in mind, we concluded that the proposed Syrian
operation, in its nature, scope, and duration, fell far short of the kinds of
engagements approved by prior Presidents under Article II. First, in contrast
with some prior deployments, the United States did not plan to employ any
U.S. ground troops, and in fact, no U.S. airplanes crossed into Syrian air-
space. Where, as here, the operation would proceed without the introduction
of U.S. troops into harm’s way, we were unlikely to be “confronted with
circumstances in which the exercise of [Congress’s] power to declare war is
effectively foreclosed.” Bosnia Deployment, 19 Op. O.L.C. at 333.
Second, the mission was sharply circumscribed. This was not a case
where the military operation served an open-ended goal. Rather, the Presi-
dent selected three military targets with the aim of degrading and destroying
the Syrian regime’s ability to produce and use chemical weapons. Mattis
Briefing (statement of Secretary Mattis) (“Earlier today, President Trump
directed the U.S. military to conduct operations in consonance with our
allies to destroy the Syrian regime’s chemical weapons research[,] develop-
ment and production capability.”); id. (“It was done on targets that we
believed were selected to hurt the chemical weapons program. We confined
it to the chemical weapons-type targets. We were not out to expand this. We

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were very precise and proportionate.”); id. (noting that “right now this is a
one-time shot”). And the strikes were planned to minimize casualties, further
demonstrating the limited nature of the operation. See DoD Briefing (state-
ment of Lt. Gen. McKenzie). Those aspects both underscored the “limited
mission” and the fact that the operation was not “aim[ed] at the conquest or
occupation of territory nor even, as did the planned Haitian intervention, at
imposing through military means a change in the character of a political
régime.” Bosnia Deployment, 19 Op. O.L.C. at 332.
Third, the duration of the planned operation was expected to be very
short. In fact, the entire operation lasted several hours, and the actual attack
lasted only a few minutes. DoD Briefing (statement of Lt. Gen. McKenzie).
Standing on its own, the attack on three Syrian chemical-weapons facili-
ties was not the kind of “prolonged and substantial military engagement”
that would amount to a war. Libya Deployment at *8. We did not, however,
measure the engagement based solely upon the contours of the first strike.
Rather, in evaluating the expected scope of hostilities, we also considered
the risk that an initial strike could escalate into a broader conflict against
Syria or its allies, such as Russia and Iran. See Haiti Deployment I, 18 Op.
O.L.C. at 179 (“In deciding whether prior Congressional authorization for
the Haitian deployment was constitutionally necessary, the President was
entitled to take into account . . . the limited antecedent risk that United States
forces would encounter significant armed resistance or suffer or inflict
substantial casualties as a result of the deployment.”). But the fact that there
is some risk to American personnel or some risk of escalation does not itself
mean that the operation amounts to a war. See Cambodian Sanctuaries,
1 Op. O.L.C. Supp. at 331; Bosnia Deployment, 19 Op. O.L.C. at 332. We
therefore considered the likelihood of escalation and the measures that the
United States intended to take to minimize that risk.
We were advised that escalation was unlikely (and reviewed materials
supporting that judgment), and we took note of several measures that had
been taken to reduce the risk of escalation by Syria or Russia. The targets
were selected because of their particular connections to the chemical-
weapons program, underscoring that the strikes sought to address the ex-
traordinary threat posed by the use of chemical weapons and did not seek to
precipitate a regime change. See DoD Briefing (statement of Ms. White)
(“This operation does not represent a change in U.S. policy, nor an attempt
to depose the Syrian regime. The strikes were [a] justified, legitimate and
proportionate response to the Syrian regime’s continued use of chemical

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weapons on its own people.”). The targets were chosen to minimize civilian
casualties, see Mattis Briefing (statement of Gen. Dunford) (“[W]e did not
select those that had a high risk of collateral damage, and specifically a high
risk of civilian casualties.”), and the strikes took place at a time that further
reduced the threat to civilians, see DoD Briefing (statement of Lt. Gen.
McKenzie) (“We also chose to strike it [at] . . . 4:00 in the morning local
time, so we weren’t trying to kill a lot of people on the objective, and so we
struck at a different time of the day.”), again reducing the likelihood that
Syria would retaliate. The targets were also chosen to minimize risk to
Russian soldiers, and deconfliction processes were used, two steps that
reduced the possibility that Russia would respond militarily. See Mattis
Briefing (statement of Gen. Dunford) (“[W]e specifically identified these
targets to mitigate the risk of Russian forces being involved, and we used
our normal deconfliction channels—those were active this week—to work
through the airspace issue and so forth.”). Given the absence of ground
troops, the limited mission and time frame, and the efforts to avoid escala-
tion, the anticipated nature, scope, and duration of these airstrikes did not
rise to the level of a “war” for constitutional purposes.

IV.

For the foregoing reasons, we concluded that the President had the con-
stitutional authority to carry out the proposed airstrikes on three Syrian
chemical-weapons facilities. The President reasonably determined that this
operation would further important national interests in promoting regional
stability, preventing the worsening of the region’s humanitarian catastrophe,
and deterring the use and proliferation of chemical weapons. Further, the
anticipated nature, scope, and duration of the operations were sufficiently
limited that they did not amount to war in the constitutional sense and there-
fore did not require prior congressional approval.

STEVEN A. ENGEL
Assistant Attorney General
Office of Legal Counsel

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