Cite as: 522 U. S. 329 (1998)
Opinion of the Court
II
States acquired primary jurisdiction over unallotted opened lands where "the applicable surplus land Act freed that land of its reservation status and thereby diminished the reservation boundaries." Solem, 465 U. S., at 467. In contrast, if a surplus land Act "simply offered non-Indians the opportunity to purchase land within established reservation boundaries," id., at 470, then the entire opened area remained Indian country. Our touchstone to determine whether a given statute diminished or retained reservation boundaries is congressional purpose. See Rosebud Sioux Tribe v. Kneip, 430 U. S. 584, 615 (1977). Congress possesses plenary power over Indian affairs, including the power to modify or eliminate tribal rights. See, e. g., Santa Clara Pueblo v. Martinez, 436 U. S. 49, 56 (1978). Accordingly, only Congress can alter the terms of an Indian treaty by diminishing a reservation, United States v. Celestine, 215 U. S. 278, 285 (1909), and its intent to do so must be "clear and plain," United States v. Dion, 476 U. S. 734, 738-739 (1986).
Here, we must determine whether Congress intended by the 1894 Act to modify the reservation set aside for the Yankton Tribe in the 1858 Treaty. Our inquiry is informed by the understanding that, at the turn of this century, Congress did not view the distinction between acquiring Indian property and assuming jurisdiction over Indian territory as a critical one, in part because "[t]he notion that reservation status of Indian lands might not be coextensive with tribal ownership was unfamiliar," Solem, 465 U. S., at 468, and in part because Congress then assumed that the reservation system would fade over time. "Given this expectation, Congress naturally failed to be meticulous in clarifying whether a particular piece of legislation formally sliced a certain parcel of land off one reservation." Ibid.; see also Hagen v. Utah, 510 U. S. 399, 426 (1994) (Blackmun, J., dissenting) ("As a result of the patina history has placed on the allotment
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