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4 said. See, e.g., Delaware Tribal Business Comm. v. Weeks, 430 U. S. 73, 84 (1977) (quoting United States v. Alcea Band of Tillamooks, 329 U. S. 40, 54 (1946) (plurality opinion)). But the formulation’s pedigree cannot make up for its vacuity. The term “plenary” is defined in one dictionary after another as “absolute.” See, e.g., New Oxford American Dictionary 1343 (3d ed. 2010); Webster’s Third New International Dictionary 1739 (2002); The Random House Dictionary of the English Language 1486 (2d ed. 1987). If we accept these definitions, what the Court says is that absolute ≠ absolute and plenary ≠ plenary, violating one of the most basic laws of logic. Surely we can do better than that.

We need not map the outer bounds of Congress’s Indian affairs authority to hold that the challenged provisions of ICWA lie outside it. We need only acknowledge that even so-called plenary powers cannot override foundational constitutional constraints. By attempting to control state judicial proceedings in a field long-recognized to be the virtually exclusive province of the States, ICWA violates the fundamental structure of our constitutional order.

In reaching this conclusion, I do not question the proposition that Congress has broad power to regulate Indian affairs. We have “consistently described” Congress’s “powers to legislate in respect to Indian tribes” as “ ‘plenary and exclusive.’ ” United States v. Lara, 541 U. S. 193, 200 (2004) (collecting cases). Reflecting this understanding, we have sanctioned a wide range of enactments that bear on Indian tribes and their members, sometimes (regrettably) without tracing the source of Congress’s authority to a particular enumerated power. See, e.g., Santa Clara Pueblo v. Martinez, 436 U. S. 49, 56–58 (1978) (modifying tribal governments’ powers of self-government); Lone Wolf v. Hitchcock, 187 U. S. 553, 565–566 (1903) (transferring tribal land). Nor do I dispute the notion that Congress has undertaken responsibilities that have been roughly analogized to those