Cite as: 530 U. S. 392 (2000)
Opinion of the Court
fense is inadmissible at this late date, and therefore we do not reach the merits of that plea. The State parties could have raised the defense in 1979 in response to the United States' motion for a supplemental decree granting additional water rights for the Fort Yuma Reservation. The State parties did not do so then, nor did they raise the objection in 1982 when Arizona II was briefed and argued.3 Unaccountably, they raised the preclusion argument for the first time in 1989, when they initiated the current round of proceedings. See Exception and Brief for State Parties 16; Motion of State Parties to Reopen Decree in Arizona v. California, O. T. 1989, No. 8 Orig., p. 6, n. 2. The State parties had every opportunity, and every incentive, to press their current preclusion argument at earlier stages in the litigation, yet failed to do so.4
3 Noting that in Arizona II we "encouraged the parties to assert their legal claims and defenses in another forum," The Chief Justice concludes that the Court probably would have declined to resolve the preclusion issue at that stage of the case even had the State parties raised it then. Post, at 423 (opinion concurring in part and dissenting in part). One can only wonder why this should be so. If this Court had held in Arizona II that the United States and the Tribe were precluded from litigating their boundary lands claims, it would have been pointless for the Court to encourage pursuit of those claims "in another forum"; further assertion of the claims in any forum would have been barred. In any event, a party generally forfeits an affirmative defense by failing to raise it even if the relevant proceeding is ultimately resolved on other grounds.
4 The dissent's observation that "the only 'pleadings' in this case were filed in the 1950's," post, at 422, is beside the point. The State parties could have properly raised the preclusion defense as early as February 1979, in their response to the United States' motion for modification of the decree, yet did not do so. See Response of the States of Arizona, California, and Nevada and the Other California Defendants to the Motion of the United States for Modification of Decree, O. T. 1978, No. 8 Orig. Alternatively, it was open to the State parties to seek leave to file a supplemental pleading "setting forth . . . occurrences or events which have happened since the date of the pleading sought to be amended." Fed. Rule Civ. Proc. 15(d). In such a supplemental pleading, and in compliance with Rule
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