WASHINGTON — The Supreme Court on Thursday ruled that nearly half of Oklahoma falls within an Indian reservation, a decision that could reshape the criminal-justice system by preventing state authorities from prosecuting offenses there that involve Native Americans.
The 5-to-4 decision, potentially one of the most consequential legal victories for Native Americans in decades, could have far-reaching implications for the 1.8 million people who live across what is now deemed “Indian Country” by the high court. The lands include much of Tulsa, Oklahoma’s second-biggest city.
The case was steeped in the United States government’s long history of brutal removals and broken treaties with Indigenous tribes, and grappled with whether lands of the Muscogee (Creek) Nation had remained a reservation after Oklahoma became a state.
Justice Neil M. Gorsuch, a Westerner who has sided with tribes in previous cases and joined the court’s more liberal members, said that Congress had granted the Creek a reservation, and that the United States needed to abide by its promises.
“Today we are asked whether the land these treaties promised remains an Indian reservation for purposes of federal criminal law,” Justice Gorsuch wrote. “Because Congress has not said otherwise, we hold the government to its word.”
Muscogee leaders hailed the decision as a hard-fought victory that clarified the status of their lands. The tribe said it would work with state and federal law enforcement authorities to coordinate public safety within the reservation.
“This is a historic day,” Principal Chief David Hill said in an interview. “This is amazing. It’s never too late to make things right.”
But Chief Justice John G. Roberts warned in a dissenting opinion that the Court had sown confusion in the state’s criminal justice system and “profoundly destabilized” the state’s powers in eastern Oklahoma.
“The State’s ability to prosecute serious crimes will be hobbled and decades of past convictions could well be thrown out,” Justice Roberts wrote. “The decision today creates significant uncertainty for the State’s continuing authority over any area that touches Indian affairs, ranging from zoning and taxation to family and environmental law.”
In a statement, Mike Hunter, Oklahoma’s attorney general, said the state and the Muscogee (Creek), Cherokee, Chickasaw, Choctaw, and Seminole Nations were working on an agreement to present to Congress and the U.S. Department of Justice addressing jurisdictional issues raised by the decision.
“We will continue our work, confident that we can accomplish more together than any of us could alone,’’ he said.
The case concerned Jimcy McGirt, a member of the Muscogee (Creek) Nation who was convicted of sex crimes against a child by state authorities in the Nation’s historical boundaries. He said that only federal authorities were entitled to prosecute him.
Mr. McGirt argued that Congress had never clearly destroyed the sovereignty of the Creek Nation over the area, covering about half the state. The solicitor general of Oklahoma took the opposite view, saying the area had never been reservation land.
McGirt v. Oklahoma, No. 18-9526, an appeal from a state court’s decision, was the Supreme Court’s second attempt to resolve the status of eastern Oklahoma.
In November 2018, the justices heard arguments in Sharp v. Murphy, No. 17-1107, which arose from the prosecution in state court of Patrick Murphy, a Creek Indian, for murdering George Jacobs in rural McIntosh County, east of Oklahoma City.
After he was sentenced to death, it emerged that the murder had taken place on what had once been Indian land. Mr. Murphy argued that only the federal government could prosecute him and that a federal law barred the imposition of the death penalty because he was an Indian.
Mr. Murphy convinced the United States Court of Appeals for the 10th Circuit, in Denver. But when the case was argued before an eight-member Supreme Court, the justices seemed divided and troubled. (Justice Neil M. Gorsuch, who had served on the 10th Circuit when it ruled on the case, recused himself.)
Instead of issuing a decision before the term ended in June 2019, the court announced it would hear another set of arguments in its current term, which started in October. That was a sign the court had deadlocked, 4 to 4.
But there was no new argument in the Murphy case, probably because it was not clear another hearing would break the deadlock. Instead, the court heard Mr. McGirt’s case, allowing the overarching issue to be settled by a nine-member court.