Indian country is in the US both by law and (obviously) geographically.
Indian country is in the U.S.
William Bradford, Chiricahua Apache. Assistant Professor of Law, Indiana University, 2002-2003, American Indian Law Review
More than half of the U.S. land mass was purchased at an average price of pennies per acre, while another 300 million acres were taken without compensation and another 700 million acres are claimed by the United States although it has taken no action to extinguish Indian title. See Russel Lawrence Barsh, Indian Land Claims Policy in the United States, 58 N.D. L. REV. 7, 8-9 (1982). Moreover, all of "Indian Country", a legal term-of-art meaning essentially lands within the territorial limits of an Indian reservation, is now either trust land owned by the United States or non-trust land permanently Indian-occupied but subject to Congressional plenary power to restrict alienation and use. See 18 U.S.C. § 1151 (2000) (defining "Indian Country"); see also Alaska v. Native Village of Venetie Tribal Gov't, 522 U.S. 234, 243-44 (1998) (noting that to qualify as "Indian Country" subject to federal jurisdiction, the trust doctrine, and limited immunity from State jurisdiction, Congress must explicitly designate lands in question, by treaty or statute, either a "reservation," "dependent Indian community," or Indian allotment). In sum, the United States owns superior title to all land within its borders, and efforts to reacquire Indian land are vigorously opposed by all levels of government. See Rebecca Tsosie, Negotiating Economic Survival: The Consent Principle and Tribal-State Compacts Under the Indian Gaming Act, 29 ARIZ. ST. L.J. 25, 80 (1997) [hereinafter Tsosie, Negotiating]. From the Indian viewpoint, non-Indians not only shamelessly "gorge themselves on the spoils of old wars" but remain unrepentant, as evinced by ongoing land seizures. Atkinson, supra note 54, at 381.
Indian country is in the U.S.
US Code, 7-1-2008,
§ 1151. Indian country defined
Except as otherwise provided in sections 1154 and 1156 of this title [18 USCS §§ 1154 and 1156], the term "Indian country", as used in this chapter [18 USCS §§ 1151 et seq.], means (a) all land within the limits of any Indian reservation under the jurisdiction of the United States Government, notwithstanding the issuance of any patent, and, including rights-of-way running through the reservation, ( all dependent Indian communities within the borders of the United States whether within the original or subsequently acquired territory thereof, and whether within or without the limits of a state, and © all Indian allotments, the Indian titles to which have not been extinguished, including rights-of-way running through the same.