The Creek Nation received assurances that their new lands in the West would be secure forever. In exchange for ceding “all their land, East of the Mississippi river,” the U. S. government agreed by treaty that “the Creek country west of the Mississippi shall be solemnly guarantied to the Creek Indians.” The government further promised that “[no] State or Territory [shall] ever have a right to pass laws for the government of such Indians, but they shall be allowed to govern themselves.” An Oklahoma state court convicted McGirt (D) of three serious sexual offenses. In postconviction proceedings, D argued that the State lacked jurisdiction to prosecute him because he is an enrolled member of the Seminole Nation of Oklahoma and his crimes took place on the Creek Reservation. D contends that only the federal government had jurisdiction to try him. No one disputes that D’s crimes were committed on lands described as the Creek Reservation in an 1866 treaty and federal statute. P claims that the land once given to the Creeks is no longer a reservation today. The Tenth Circuit has reached the opposite conclusion. The Supreme Court granted certiorari.