dissenting.
In 1942, the Sioux Tribe filed a petition for certiorari requesting this Court to review the Court of Claims’ ruling that Congress had not unconstitutionally taken the Black Hills in 1877, but had merely exchanged the Black Hills for rations and grazing lands — an exchange Congress believed to be in the best interests of the Sioux and the Nation. This Court declined to review that judgment. Sioux Tribe v. United States, 97 Ct. Cl. 613 (1942), cert. denied, 318 U. S. 789 (1943). Yet today the Court permits Congress to reopen that judgment which this Court rendered final upon denying certiorari in 1943, and proceeds to reject the 1942 Court of Claims’ factual interpretation of the events in 1877. I am convinced that Congress may not constitutionally require the Court of Claims to reopen this proceeding, that there is no judicial principle justifying the decision to afford the respondents an additional *425opportunity to litigate the same claim, and that the Court of Claims’ first interpretation of the events in 1877 was by all accounts the more realistic one. I therefore dissent.
I
In 1920, Congress enacted a special jurisdictional Act, ch. 222, 41 Stat. 738, authorizing the Sioux Tribe to submit any legal or equitable claim against the United States to the Court of Claims. The Sioux filed suit claiming that the 1877 Act removing the Black Hills from.the Sioux territory was an unconstitutional taking. In Sioux Tribe v. United States, supra, the Court of Claims considered the question fully and found that the United States had not taken the Black Hills from the Sioux within the meaning of the Fifth Amendment. It is important to highlight what that court found. It did not decide, as the Court today suggests, that it merely lacked jurisdiction over the claim presented by the Sioux. See ante, at 384. It found that under the circumstances, presented in 1877, Congress attempted to improve the situation of the Sioux and the Nation by exchanging the Black Hills for 900,000 acres of grazing lands and rations for as long as they should be needed. The court found that although the Government attempted to keep white settlers and gold prospectors out of the Black Hills territory, these efforts were unsuccessful. The court concluded that this situation was such that the Government “believed serious conflicts would develop between the settlers and the Government, and between the settlers and the Indians.” 97 Ct. CL, at 659. It was also apparent to Congress that the Indians were still “incapable of supporting themselves.” Ibid.
The court found that the Government therefore embarked upon a course designed to obtain the Indians’ agreement to sell the Black Hills and “endeavored in every way possible during 1875 and 1876 to arrive at a mutual agreement with the Indians for the sale. . . .” Id., at 681. Negotiation having failed, Congress then turned to design terms for the ac*426quisition of the Black Hills which it found to be in the best interest of both the United States and the Sioux. The court found that pursuant to the 1877 agreement, Congress provided the Indians with more than $43 million in rations as well as providing them with 900,000 acres of needed grazing lands. Thus the court concluded that “the record shows that the action taken was pursuant to a policy which the Congress deemed to be for the interest of the Indians and just to both parties.” Id., at 668. The court emphasized:
“[T]he Congress, in an act enacted because of the situation encountered and pursuant to a policy which in its wisdom it deemed to be in the interest and for the benefit and welfare of the . . . Sioux Tribe, as well as for the necessities of the Government, required the Indians to sell or surrender to the Government a portion of their land and hunting rights on other land in return for that which the Congress, in its judgment, deemed to be adequate consideration for what the Indians were required to give up, which consideration the Government was not otherwise under any legal obligation to pay.” Id., at 667.
This Court denied certiorari. 318 U. S. 789 (1943).
During the course of further litigation commencing in 1950, the Sioux again resubmitted their claim that the Black Hills were taken unconstitutionally. The Government pleaded res judicata as a defense. The Court of Claims held that res judicata barred relitigation of the question since the original Court of Claims decision had clearly held that the appropriation of the Black Hills was not a taking because Congress in “exercising its plenary power over Indian tribes, took their land without their consent and substituted for it something conceived by Congress to be an equivalent.” United States v. Sioux Nation, 207 Ct. Cl. 234, 243, 518 F. 2d 1298, 1303 (1975). The court found no basis for relieving the Sioux from the bar of res judicata finding that the disability “is not lifted if a later court disagrees with a prior one.” Id., at 244, *427518 F. 2d, at 1303. The court thus considered the equities entailed by the application of res judicata in this cáse and held that relitigation was unwarranted. Again, this Court denied certiorari. 423 U. S. 1016 (1975).
Congress then passed another statute authorizing, the Sioux to relitigate their taking claim in the Court of Claims. 92 Stat. 153. The statute provided that the Court of Claims “shall review on the merits” the Sioux claim that there was a taking and that the Court “shall determine that issue de novo.” (Emphasis added.) Neither party submitted additional evidence and the Court of Claims decided the case on the basis of the record generated in the 1942 case and before the Commission. On the basis of that same record, the Court of Claims has now determined that the facts establish that Congress did not act in the best interest of the Sioux, as the 1942 court found, but arbitrarily appropriated the Black Hills without affording just compensation. This Court now embraces this second, latter-day interpretation of the facts in 1877.
H-i
Although the Court refrains from so boldly characterizing its action, it is obvious from these facts that Congress has reviewed the decisions of the Court of Claims, set aside the judgment that no taking of the Black Hills occurred, set aside the judgment that there is no cognizable reason for relitigating this claim, and ordered a new trial. I am convinced that this is nothing other than an exercise of judicial power reserved to Art. Ill courts that may . not be performed by the Legislative Branch under its Art. I authority.
Article III vests “the judical Power ... of the United States” in federal courts. Congress is vested by Art. I with , legislative powers, and may not itself exercise an appellate-!! ¡ type review of judicial judgments in order to alter their.f ¡terms, or to order new trials of cases already decided. The, judges in Hayburn’s Case, 2 Dall. 409, 413, n. 4 (1792), stated *428that “no decision of any court of the United States can, under any circumstances, in our opinion, agreeable to the Constitution, be liable to a reversion, or even suspension, by the Legislature itself, in whom no judicial power of any kind appears to be vested.” We have interpreted the decision in United States v. Klein, 13 Wall. 128 (1872), as having “rested upon the ground that . . . Congress was without constitutional authority to control the exercise of . . . judicial power . . . by requiring this Court to set aside the judgment of the Court of Claims” and as holding that Congress may not “require a new trial of the issues . . . which the Court had resolved against [a party] .” Pope v. United States, 323 U. S. 1, 8, 9 (1944).
This principle was again applied in United States v. O’Grady, 22 Wall. 641, 647 (1875), where the Court refused to legitimize a congressional attempt to revise a final judgment rendered by the Court of Claims finding that such judgments “are beyond all doubt the final determination of the matter in controversy; and it is equally certain that the judgments of the Court of Claims, where no appeal is taken to this court, are, under existing laws, absolutely conclusive of the rights of the parties, unless a new trial is granted by that court. . . .” (Emphasis added.) The Court further found that there is only one Supreme Court and “[i]t is quite clear that Congress cannot subject the judgments of the Supreme Court to the re-examination and revision of any other tribunal or any other department of the government.” Id., at 648. See also Chicago & Southern Air Lines, Inc. v. Waterman S. S. Corp., 333 U. S. 103 (1948). Congress has exceeded the legislative boundaries drawn by these cases and the Constitution and exercised judicial power in a case already decided by effectively ordering a new trial.
The determination of whether this action is an exercise of legislative or judicial power is of course one of characterization. The fact that the judicial process is affected by an Act of Con*429gress is not dispositive since many actions which this Court has clearly held to be legitimate exercises of legislative authority do have an effect on the judiciary and its processes. Congress may legitimately exercise legislative powers in the regulation of judicial jurisdiction; and it may, like other litigants, change the import of a final judgment by establishing new legal rights after the date of judgment, and have an effect on the grounds available for a court’s decision by waiving available defenses. But as the Court apparently concedes, Congress may not, in the name of those legitimate actions, review and set aside a final judgment of an Art. Ill court, and order the courts to rehear an issue previously decided in a particular case.
The Court relies heavily on the fact that Congress was acting pursuant to its power to pay the Nation’s debts. No doubt, Congress has broad power to do just that, but it may do so only through the exercise of legislative, not judicial powers. Thus the question must be, not whether Congress was attempting to pay its debts through this Act, but whether it attempted to do so by means of judicial power. The Court suggests that the congressional action in issue is justified as either a permissible regulation of jurisdiction, the creation of a new obligation, or the mere waiver of a litigant’s right. These alternative nonjudicial characterizations of the congressional action, however, are simply unpersuasive.
A
The Court first attempts to categorize this action as a permissible regulation of jurisdiction stating that all Congress has done is to “provid[e] a forum so that a new judicial review of the Black Hills claim could take place.” But that is the essence of an appellate or trial court decision ordering a new trial. While Congress may regulate judicial functions it may not itself exercise them. Admittedly, it is not always readily apparent whether a particular action constitutes the assignment or the exercise of a judicial function since *430the assignment of some functions is inherently judicial — such as assigning the trial court the task of rehearing a case because of error. The guidelines identified in our opinions, however, indicate that while Congress enjoys broad authority to regulate judicial proceedings in the context of a class of cases, Johannessen v. United States, 225 U. S. 227 (1912), when Congress regulates functions of the judiciary in a pending case it walks the line between judicial and legislative authority, and exceeds that line if it sets aside a judgment or orders retrial of a previously adjudicated issue. United States v. Klein, supra, at 145; Pope v. United States, supra.
By ordering a rehearing in a pending case, Congress does not merely assign a judicial function, it necessarily reviews and sets aside an otherwise final adjudication; actions which this Court concedes Congress cannot permissibly take under the decisions of this Court. Ante, at 391-392. The Court concludes that no “review” of the Court of Claims decisions (and our denials of certiorari) has occurred, and that the finality of the judgments has not been disturbed, principally because Congress has not dictated a rule of decision that must govern the ultimate outcome of the adjudication. The fact that Congress did not dictate to the Court of Claims that a particular result be reached does not in any way negate the fact it has sought to exercise judicial power. This Court and other appellate courts often reverse a trial court for error without indicating what the result should be when the claim is heard again.
It is also apparent that Congress must have “reviewed” the merits of the litigation and concluded that for some reason, the Sioux should have a second opportunity to air their claims. The order of a new trial inevitably reflects some measure of dissatisfaction with at least the manner in which the original claim was heard. It certainly seems doubtful that Congress would grant a litigant a new trial if convinced that the litigant had been fairly heard in the first instance. Unless Congress is assuming that there were deficiencies in the prior judicial *431proceeding, why would it see fit to appropriate public money to have the claim heard once again? It would seem that Congress did not find the opinions of the Court of Claims fully persuasive. But it is not the province of Congress to judge the persuasiveness of the opinions of federal courts — that is the judiciary’s province alone. It is equally apparent that Congress has set aside the judgments of the Court of Claims. Previously those judgments were dispositive of the issues litigated in them; Congress now says that they are not. The action of Congress cannot be justified as the regulation of the jurisdiction of the federal courts because it seeks to provide a forum for the purposes of reviewing a previously final judgment in a pending case.
B
The action also cannot be characterized and upheld as merely an exercise of a litigant’s power to change the effect of a judgment by agreeing to obligations beyond those required by a particular judgment. This Court has clearly never found that the judicial power is encroached upon because Congress seeks to change the law after a question has been adjudicated. See, e. g., Pennsylvania v. Wheeling & Belmont Bridge Co., 18 How. 421 (1856); Hodges v. Snyder, 261 U. S. 600 (1923). This is a recognition of the right of every litigant to pay his adversary more than the court says is required if he so chooses. Congress, acting under its spending powers, is, like an individual, entitled to enlarge its obligations after the court has adjudicated a question. The decision in Pope v. United States, 323 U. S. 1 (1944), clearly rests upon this distinction.
But here Congress has made no change in the applicable law. It has not provided, as our opinions make clear it could have, that the Sioux should recover for all interest on the value of the Black Hills. Counsel for respondents in fact stated at oral argument that he could not persuade Congress “to go that far.” Congress has not changed the rule of law, it simply directed the judiciary to try again. Congress may not attempt *432to shift its legislative responsibilities and satisfy its constituents by discarding final judgments and ordering new trials.
C
The Court also suggests that the congressional action is but a “mere waiver” of a defense within a litigant’s prerogative. Ante, at 407. Congress certainly is no different from other litigants in this regard, and if the congressional action in this case could convincingly be construed as having an effect no greater than an ordinary litigant’s waiver, I certainly would not object that Congress was exercising judicial power. But it is apparent that the congressional action in issue accomplished far more than a litigant’s waiver. Congress clearly required the Court of Claims to hear the case in full, and only if a waiver of res judicata by a litigant would always impose an obligation on a federal court to rehear such a claim, could it be said that Congress has exercised the power of a litigant rather than the power of a legislature.
While res judicata is a defense which can be waived, see Fed. Rule Civ. Proc. 8 (c), if a court is on notice that it has previously decided the issue presented, the court may dismiss the action sua sponte, even though the defense has not been raised. See Hedger Transportation Corp. v. Ira S. Bushey & Sons, 186 F. 2d 236 (CA2 1951); Evarts v. Western Metal Finishing Co., 253 F. 2d 637, 639, n. 1 (CA9), cert. denied, 358 U. S. 815 (1958); Scholla v. Scholla, 92 U. S. App. D. C. 9, 201 F. 2d 211 (1953); Hicks v. Holland, 235 F. 2d 183 (CA6), cert. denied, 352 U. S. 855 (1956). This result is fully consistent with the policies underlying res judicata: it is not based solely on the defendant’s interest in avoiding the burdens of twice defending a suit, but is also based on the avoidance of unnecessary judicial waste. Commissioner v. Sunnen, 333 U. S. 591, 597 (1948); Blonder-Tongue Laboratories, Inc. v. University of Illinois Foundation, 402 U. S. 313, 328 (1971); Parklane Hosiery Co. v. Shore, 439 U. S. 322 (1979). The Court of Claims itself has indicated that it would not engage *433in reconsideration of an issue previously decided by the Court of Claims without substantial justification:
“It is well to remember that res judicata and its offspring, collateral estoppel, are not statutory defenses; they are defenses adopted by the courts in furtherance of prompt and efficient administration of the business that comes before them. They are grounded on the theory that one litigant cannot unduly consume the time of the court at the expense of other litigants, and that, once the court has finally decided an issue, a litigant cannot demand that it be decided again.” Warthen v. United States, 157 Ct. Cl. 798, 800 (1962).
It matters not that the defendant has consented to the relitigation of the claim since the judiciary retains an independent interest in preventing the misallocation of judicial resources and second-guessing prior panels of Art. Ill judges when the issue has been fully and fairly litigated in a prior proceeding. Since the Court of Claims found in this case that there was no adequate reason for denying res judicata effect after the issue was raised and the respondents were given an opportunity to demonstrate why res judicata should not apply, it is clear that the issue has been heard again only because Congress used its legislative authority to mandate a rehearing. The Court of Claims apparently acknowledged that this in fact was the effect of the legislation, for it did not state that readjudication was the product of a waiver, but rather that through its decision the court “carried out the obligation imposed upon us in the 1978 jurisdictional statute.” (Emphasis added.)
Nor do I find this Court’s decision in Cherokee Nation v. United States, 270 U. S. 476 (1926), dispositive. Again, in Cherokee Nation, the Court was asked to consider and decide a question not previously adjudicated by the Court of Claims. The Court stated that the theory of interest presented in the second adjudication was not “presented either to the Court *434of Claims or to this Court. It is a new argument not before considered.” Id,., at 486. Thus even Cherokee Nation did not involve congressionally mandated" judicial re-examination of a question previously decided by an Art. Ill court.
Here, in contrast, the issue decided is identical to that decided in 1942. It is quite clear from a comparison of the 1942 decision of.the Court of Claims and the opinion of the Court today that the only thing that has changed is an interpretation of the events which occurred in 1877. The Court today concludes that the facts in this case “would not lead one to conclude that the Act effected 'a mere change in the form of investment of Indian tribal property.’ ” Ante, at 413. But that is precisely what the Court of Claims found in 1942. See supra, at 425-426. There has not even been a change in the law, for the Court today relies on decisions rendered long before the Court of Claims decision , in 1942. It is the view of history, and not the law, which has evolved. See infra, at 434-437. The decision is thus clearly nothing more than a second interpretation of the precise factual question decided in 1942. As the dissenting judges in the Court of Claims aptly stated: “The facts have not changed. We have been offered no new evidence.” 220 Ct. Cl. 442, 489, 601 F. 2d 1157, 1184.
It is therefore apparent that Congress has accomplished more than a private litigant’s attempted waiver, more than legislative control over the general jurisdiction of the federal courts, and more than the establishment of a new rule of law for a previously decided case. What Congress has done is uniquely judicial. It has reviewed a prior decision of an Art. Ill court, eviscerated the finality of that judgment, and ordered a new trial in a pending case.
Ill
Even if I could countenance the Court’s decision to reach the merits of this case, I also think it has erred in rejecting the 1942 court’s interpretation of the facts. That court *435rendered a very persuasive account of the congressional enactment. See supra, at 425-426. As the dissenting judges in the Court of Claims opinion under review pointedly stated: “The majority’s view that the rations were not consideration for the Black Hills in untenable. What else was the money for?” 220 a. Cl., at 487, 601 F. 2d, at 1183.
I think the Court today rejects that conclusion largely on the basis of a view of the settlement of the American West which is not universally shared. There were undoubtedly greed, cupidity, and other less-than-admirable tactics employed by the Government during the Black Hills episode in the settlement of the West, but the Indians did not lack their share of villainy either. It seems to me quite unfair to judge by the light óf “revisionist” historians or the mores of another era actions that were taken under pressure of time more than a century ago.
Different historians, not writing for the purpose of having their conclusions or observations inserted in the reports of congressional committees, have taken different positions than those expressed in some of the materials referred to in the Court’s opinion. This is not unnatural, since history, no more than law, is not an exact (or for that matter an inexact) science.
But the inferences which the Court itself draws from the letter from General Sheridan to General Sherman reporting on a meeting between the former with President Grant, the Secretary of the Interior, and the Secretary of War, as well as other passages in the Court’s opinion, leave a stereotyped and one-sided impression both of the settlement regarding the Black Hills portion of the Great Sioux Reservation and of the gradual expansion of the National Government from the Proclamation Line of King George III in 1763 to the Pacific Ocean.
Ray Billington, a senior research associate at the Huntington Library in San Marino, Cal., since 1963, and a respected student of the settlement of the American West, em*436phasized in his introduction to the book Soldier and Brave (National Park Service, U. S. Dept, of the Interior, 1963) that the confrontations in the West were the product of a long history, not a conniving Presidential administration:
“Three centuries of bitter Indian warfare reached a tragic climax on the plains and mountains of America’s Far West. Since the early seventeenth century, when Chief Opechancanough rallied his Powhatan tribesmen against the Virginia intruders on their lands, each advance of the frontier had been met with stubborn resistance. At times this conflict flamed into open warfare: in King Phillips’ rebellion against the Massachusetts Puritans, during the French and Indian Wars of the eighteenth century, in Chief Pontiac’s assault on his new British overlords in 1763, in Chief Tecumseh’s vain efforts to hold back the advancing pioneers of 1812, and in the Black Hawk War. . . .
“. . . In three tragic decades, between 1860 and 1890, the Indians suffered the humiliating defeats that forced them to walk the white man’s road toward civilization. Few conquered people in the history of mankind have paid so dearly for their defense of a way of life that the march of progress had outmoded.
“This epic struggle left its landmarks behind, as monuments to the brave men, Indian and white, who fought and died that their manner of living might endure.” Id., at xiii-xiv.
Another history highlights the cultural differences which made conflict and brutal warfare inevitable:
“The Plains Indians seldom practiced agriculture or other primitive arts, but they were fine physical specimens; and in warfare, once they had learned the use of the rifle, [were] much more formidable than the Eastern tribes who had slowly yielded to the white man. Tribe warred with tribe, and a highly developed sign language *437was the only means of intertribal communication. The effective unit was the band or village of a few hundred souls, which might be seen in the course of its wanderings encamped by a watercourse with tipis erected; or pouring over the plain, women and children leading dogs and packhorses with their trailing travois, while gaily dressed braves loped ahead on horseback. They lived only for the day, recognized no rights of property, robbed or killed anyone if they thought they could get away with it, inflicted cruelty without a qualm, and endured torture without flinching.” S. Morison, The Oxford History of the American People 539-540 (1965).
That there was tragedy, deception, barbarity, and virtually every other vice known to man in the 300-year history of the expansion of the original 13 Colonies into a Nation which now embraces more than three million square miles and 50 States cannot be denied. But in a court opinion, as a historical and not a legal matter, both settler and Indian are entitled to the benefit of the Biblical adjuration: “Judge not, that ye be not judged.”