The Supreme Court ruled Thursday in a 5-4 decision that when Oklahoma became a state in 1906, Creek tribal lands within its borders never lost their reservation status, and therefore under the Major Crimes Act criminal cases stemming from that area can only be handled in federal courts, not state ones.

The case was brought by Jimcy McGirt, a member of the Creek Nation, who claimed he is entitled to a new trial because a state criminal case against him in which he was convicted of first-degree rape of a child is based on a crime he was accused of committing on Creek land in eastern Oklahoma. The state argued that the land lost its reservation status, even though there was no explicit action by Congress. The Supreme Court rejected that argument.

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"The federal government promised the Creek a reservation in perpetuity," Justice Neil Gorsuch wrote in the court's opinion. "Over time, Congress has diminished that reservation. It has sometimes restricted and other times expanded the Tribe’s authority. But Congress has never withdrawn the promised reservation."

The decision threatens the validity of more than a century's worth of state criminal convictions, something Chief Justice John Roberts noted in his dissenting opinion.

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"Across this vast area, the state’s ability to prosecute serious crimes will be hobbled and decades of past convictions could well be thrown out," Roberts wrote, adding that the majority's opinion "has profoundly destabilized the governance of eastern Oklahoma."

Roberts rejected the idea that the land retains its reservation status, saying it was rescinded "in a series of statutes leading up to Oklahoma statehood at the turn of the 19th century."