OPINION AND ORDER
KLEINFELD, Circuit Judge:The Navy and environmental advocacy organizations have battled for years about whether Navy training using sonar is too harmful to the environment, particularly whales. The Navy uses something called medium frequency active sonar, which basically bounces a loud noise off the hulls of extremely quiet submarines to detect then-presence. The loud noise may be quite harmful to whales and other marine mammals. In a previous round of this litigation, the district court had approved a settlement that allowed Navy sonar training to proceed, but required mitigation “measures.” The measures consisted of such precautions as requiring some sailors to be on deck looking for whales, and reducing the decibel level when whales were present, weather prevented seeing whether any whales were around, or “surface ducting” would let the noise carry more.1
In this round of the litigation, the Navy proposed to use medium frequency active sonar in training exercises off the coast of Southern California without mitigation measures. The record does not show why the Navy does not propose the mitigation measures it has previously used. The district court issued a preliminary injunction under the National Environmental Policy Act2 and the Coastal Zone Management Act.3 The injunction prohibits all use of medium frequency active sonar off the coast of Southern California during the fourteen large training exercises from 2007 to 2009.4 The district court did not tailor the injunction in any way, such as by requiring the mitigation measures it had found sufficient before. The district court offers no more explanation of why the training could not be allowed to proceed with mitigation measures than the Navy does for why it does not want to commit itself to using mitigation measures. There is no explanation in the record for the breadth of the Navy’s position or of the district court’s injunction.
Medium frequency active sonar has proven to be the most effective method of detecting quiet-running diesel-electric submarines by emitting sound underwater at extreme pressure levels. The 2007 to 2009 exercises at issue were designed to train the full array of land, sea, undersea, and air components of the Pacific Fleet to perform successfully in complex, coordinated combat missions. An advocacy group, the Natural Resources Defense Council, and four other plaintiffs filed this action against the Navy, alleging that by finding *862no significant environmental impact after an environmental assessment, instead of preparing a full environmental impact statement, and by concluding that there was no effect on coastal resources, the Navy violated the National Environmental Policy Act,5 the Endangered Species Act,6 the Administrative Procedures Act,7 and the Coastal Zone Management Act.8 Finding that the plaintiffs had demonstrated a high probability of success on the merits of all claims save the Endangered Species Act claim and a “near certainty” of irreparable harm to the environment, the district court enjoined the Navy from using medium frequency sonar during the fourteen challenged SOCAL training exercises.9 The Navy filed an emergency motion for stay of the injunction pending appeal, which we grant.
Two standards affect our determination, the standard applicable to district courts for preliminary injunctions, and the standard for appellate courts for stays pending appeal. The district court must apply a four part standard, or a sliding scale. What is critical to our review for abuse of discretion10 is that the district court must consider not only the possibility of irreparable harm, but also, in appropriate cases, the public interest. The public interest is not the same thing as the hardship to the party against whom the injunction was issued. Balance of hardships is the third factor, and the public interest is the fourth factor. They are separate:
Under the traditional test, a plaintiff must show: (1) a strong likelihood of success on the merits, (2) the possibility of irreparable injury to plaintiff if preliminary relief is not granted, (3) a balance of hardships favoring the plaintiff, and (4) advancement of the public interest (in certain cases). The alternative test requires that a plaintiff demonstrate either a combination of probable success on the merits and the possibility of irreparable injury or that serious questions are raised and the balance of hardships tips sharply in his favor. These two formulations represent two points on a sliding scale in which the required degree of irreparable harm increases as the probability of success decreases. They are not separate tests but rather outer reaches of a single continuum.11
The district court was required to consider, not only “balance of hardships” as between the plaintiffs and the Navy as an Executive Branch agency, but also the “public interest” in having a trained and effective Navy. We customarily give considerable deference to the Executive Branch’s judgment regarding foreign policy and national defense.12
*863The Supreme Court in Hilton v. Braunskill13 articulated the similar standard appellate courts are required to apply for stays of civil judgments pending appeal.14 This standard requires us to consider “where the public interest lies” separately from and in addition to “whether the applicant [for stay] will be irreparably injured absent a stay:” 15
The factors regulating issuance of a stay [include]: (1) whether the stay applicant has made a strong showing that he is likely to succeed on the merits; (2) whether the applicant will he irreparably injured absent a stay; (3) whether issuance of the stay will substantially injure the other parties interested in the proceeding; and (4) where the public interest lies.16
Hilton emphasizes that even “failing” a strong likelihood of success on the merits, the party seeking a stay may be entitled to prevail if it can demonstrate a “substantial case on the merits” and the second and fourth factors militate in its favor.17 The district court did not give serious consideration to the public interest factor. All our dissenting colleague can come up with is an oblique reference in the oral discussion preceding the order. All the order contains is a conclusory remark about “the harm the Defendants will suffer;” That is the third factor, not the fourth. There is not a word in the order about the interest of the public, as distinguished from the interest of the Navy, in war preparedness: 18
The Court is also satisfied that the balance of hardships tips in favor of granting an injunction, as the harm to the environment, Plaintiffs, and public interest outweighs the harm that Defendants would incur if prevented from using MFA sonar, absent the use of effective mitigation measures, during a subset of their regular activities in one part of one state for a limited period.19
The reference to “public interest” by the district court extends only to the interest in protecting marine mammals, especially beaked whales, not the interest in national defense.
The public does indeed have a very considerable interest in preserving our natural environment and especially relatively scarce whales.20 But it also has an interest in national defense. We are currently engaged in war, in two countries. There are no guarantees extending from 2007 to 2009 or at any other time against other countries deciding to engage us, or our determining that it is necessary to engage *864other countries. The safety of the whales must be weighed, and so must the safety of our warriors. And of our country.
Our dissenting colleague also argues that “the Navy is free to proceed at any time with its MFA sonar training exercises outside the SOCAL area that are similar to conditions in the SOCAL area.” The environmental assessment,21 though, explains that “this particular location” matters.22 According to that document, “[t]here is no duplicative location where land, sea, undersea and airspace assets are controlled by military authorities that allow full play and training by THIRD Fleet operational actors.”23 The environmental assessment further explains that none of the potential alternative locations, including Alaska and Hawaii, “provide the full complement of range infrastructure necessary to conduct typical, realistic, coordinated COMPTUEX and JTFEX training.”24 Although one-time training operations have been conducted off Alaska and Hawaii, the environmental assessment says that “routine usage of these training areas for the major exercises is infeasible.”25 Because the record offers no support for it, we respectfully disagree with our dissenting colleague’s implication that the Navy ought to do whatever it needs to do someplace other than off the coast of Southern California.
The district court did not explain why a broad, absolute injunction against the use of the medium frequency active sonar in these complex training exercises for two years was necessary to avoid irreparable harm to the environment. The district court’s previous approval of similar exercises subject to mitigation measures requires some explanation, which we cannot find in the order granting the injunction, for why that is no longer sufficient. Nor does the Navy explain why it no longer proposes to use these mitigation measures, a factor that militates against its probability of full success on the merits in district court. On appeal, though, because of the breadth of the injunction, and the district court’s failure to consider the fourth factor, the Navy’s probability of at least partial success on the merits is high. At the least, the Navy presents a “substantial” 26 case on appeal, and the “second and fourth factors”27 militate in its favor. Applying independently on appeal our duty under Hilton28 to consider the fourth factor, the public interest, we are obligated to grant a stay pending appeal of the preliminary injunction.
Our conclusion is limited to what is before us, a district court injunction absolutely prohibiting the Navy’s use of medium frequency active sonar in its training program rather than tailoring the injunction with mitigation measures. We do not suggest whether an injunction allowing the exercises but subjecting them to mitigation measures might lead to a different result, because no such injunction is before us. The environmental assessment says that there would be no significant environmental impact if the Navy used lookouts for *865marine mammals, made binoculars available to the lookouts, and reduced the noise during “surface ducting” conditions or when it was so foggy that the lookouts would not be able to see marine mammals.
Expeditious determination of this appeal can eliminate a great deal of the risk to both our country and to marine wildlife. Accordingly, we order expedited briefing and calendaring of this appeal. The provisions of Ninth Circuit rule 31 — 2.2(a) shall not apply to this appeal. A briefing schedule is set out in a separate order. Any motions to extend time to file the briefs will be strongly disfavored.
The Navy’s emergency motion to stay the preliminary injunction entered by the district court on August 7, 2007 is GRANTED.29
. See NRDC v. Winter, Settlement Agreement, CV-06-4131-FMC (C.D.Cal. July 7, 2006).
. 42 U.S.C. §§ 4321-4347.
. 16 U.S.C. § 1451 etseq.
. NRDC v. Winter, Order, CV-07-00335-FMC at 20, 2007 WL 2481037 (C.D.Cal. Aug. 7, 2007).
. 42 U.S.C. §§ 4321-4347.
. 16 U.S.C. § 1536.
. 5 U.S.C. § 551 etseq.
. 16 U.S.C. § 1451 etseq.
. NRDC v. Winter, Order, CV-07-00335-FMC, 2007 WL 2481037 (C.D.Cal. Aug. 7, 2007).
. See Sports Form, Inc. v. United Press Int’l, Inc., 686 F.2d 750, 752 (9th Cir.1982).
. Taylor v. Westly, 488 F.3d 1197, 1200 (9th Cir.2007) (emphasis added).
. E.g., Dep't of Navy v. Egan, 484 U.S. 518, 529, 108 S.Ct. 818, 98 L.Ed.2d 918 (1988) (noting that "unless Congress specifically has provided otherwise, courts traditionally have been reluctant to intrude upon the authority of the Executive in military and national security affairs.’’). The dissent argues that Egan was "not an environmental case” and that it relied heavily on "the President’s authority as Commander in Chief.” True, NEPA applies to the Navy, but that is not a distinction that makes a difference. There is no exception to the President’s authority as Commander in Chief for environmental cases.
. Hilton v. Braunskill, 481 U.S. 770, 107 S.Ct. 2113, 95 L.Ed.2d 724 (1987).
. The dissent accurately notes that Hilton involved a stay of a writ of habeas corpus, but erroneously argues that the Hilton standard would therefore not apply to an environmental case. The Court in Hilton says that it is using "the traditional standards governing stays of civil judgments” to interpret the rules for stays of writs of habeas corpus, id. at 774, 107 S.Ct. 2113, and "the factors regulating the issuance of a stay are generally the same,” id. at 776, 107 S.Ct. 2113. That leaves no room for the dissent’s position that they are not "generally the same” or that, as the dissent says, "Hilton does not apply here.”
. Id. at 776, 107 S.Ct. 2113.
. Id. (emphasis added); see Fed.R.Civ.P. 62(c); Fed. R.App. P. 8(a).
. Hilton v. Braunskill, 481 U.S. 770, 778, 107 S.Ct. 2113, 95 L.Ed.2d 724 (1987).
. We reviewed classified documents submitted by the Navy to the district court and considered them in coming to our decision.
. NRDC v. Winter, Order, CV-07-00335-FMC at 19, 2007 WL 2481037 (C.D.Cal. Aug. 7, 2007).
. The main argument of the dissent is that NEPA applies to the Navy. We do not disagree.
. United States Navy, Composite Training Unit Exercises and Joint Task Force Exercises, Environmental Assessment/Overseas Environmental Assessment, Final, available at http://www.navydocuments.com/documents/ COMPTUEX-JTFEXEA-OEA.pdf (February 2007) (last visited Aug. 29, 2007).
. Id. at 2-32.
. Id.
. Id. at 2-33.
. Id.
. See Hilton v. Braunskill, 481 U.S. 770, 778, 107 S.Ct. 2113, 95 L.Ed.2d 724 (1987).
. See id.
. Id. at 776, 107 S.Ct. 2113.
. Natural Resources Defense Council's motion to strike the "Unclassified Declaration Addendum of David Yoshihara,” submitted by Defendants-Appellants with their reply brief, is GRANTED, because it contains new evidence not presented to the district court. See Fed. R.App. P. 10(a). All other motions are referred for consideration to the merits panel.