The case was Rosebud Sioux Tribe v. Kneip. The question was jurisdictional. Did the State of South Dakota have the right to prosecute a Native American for a crime committed on land that was originally part of the Rosebud Reservation? The land in question had been carved out by a series of congressional acts between 1904 and 1910. These acts opened parcels to white settlement.
The Court, in a 6-3 decision delivered on April 5, 1977, said yes. The state had jurisdiction. The reasoning was technical, a matter of statutory interpretation. The majority opinion, written by Justice William Rehnquist, concluded that Congress had clearly intended to 'diminish' the reservation through these allotment acts. Therefore, the affected lands were no longer 'Indian country' for legal purposes.
The power of the decision lies in its quiet language. It did not involve soldiers or forced marches. It was a review of paperwork. By analyzing maps and legislative history, the Court ratified a process of erosion. Sovereignty was not violently seized but administratively disestablished. The reservation's boundaries, once guaranteed by treaty, were rendered permeable by statute.
The dissent, written by Justice Thurgood Marshall, called the outcome a 'great injustice.' He argued the acts were about land ownership, not political boundaries. The majority saw a change in lines on a map. The dissent saw the dissolution of a promise. The ruling set a precedent. It established that congressional intent, often inferred from century-old documents, could legally unravel the geographic foundation of tribal authority. It was sovereignty undone not by conquest, but by ledger.