476
Opinion of the Court
emptive effect of § 301 of the Labor Management Relations Act, 1947). Although federal preemption is ordinarily a defense, "[o]nce an area of state law has been completely preempted, any claim purportedly based on that pre-empted state-law claim is considered, from its inception, a federal claim, and therefore arises under federal law." Caterpillar, 482 U. S., at 393.
B
Petitioners' complaint sought recognition and enforcement of a mortgage. The dispute involved Louisiana parties only, and petitioners relied exclusively on Louisiana law. Respondents defended their removal of the case from state court to federal court on the ground that petitioners' action was precluded, as a matter of federal law, by the earlier Bankruptcy Court orders. We now explain why the removal was improper.
Under the doctrine of claim preclusion, "[a] final judgment on the merits of an action precludes the parties or their privies from relitigating issues that were or could have been raised in that action." Moitie, 452 U. S., at 398; see also Baker v. General Motors Corp., ante, at 233, n. 5 ("a valid final adjudication of a claim precludes a second action on that claim or any part of it"). Claim preclusion (res judicata), as Rule 8(c) of the Federal Rules of Civil Procedure makes clear, is an affirmative defense. See also Blonder-Tongue Laboratories, Inc. v. University of Ill. Foundation, 402 U. S. 313, 350 (1971) ("Res judicata and collateral estoppel [issue preclusion] are affirmative defenses that must be pleaded." (italics omitted)).
A case blocked by the claim preclusive effect of a prior federal judgment differs from the standard case governed by a completely preemptive federal statute in this critical respect: The prior federal judgment does not transform the plaintiff's state-law claims into federal claims but rather extinguishes them altogether. See Commissioner v. Sunnen,
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