The Mescalero Apache Tribe in New Mexico (Tribe) (plaintiff) provided fishing and hunting activities for Indians and non-Indians on the reservation. Under tribal ordinances approved by the federal government, non-Indians were permitted to hunt and fish under a license issued by the Tribe. The State of New Mexico (defendant) claimed concurrent jurisdiction to enforce state fishing and hunting regulations against non-Indians on the reservation. New Mexico began to arrest non-Indians who possessed game taken legally on the reservation, but taken illegally under state law. The Tribe sued to enjoin New Mexico from enforcing state regulations on the reservation. The trial and appellate courts ruled in favor of the Tribe. The United States Supreme Court granted certiorari.