SCOTUS holds in McGirt, via 5-4 vote with Justice Gorsuch authoring majority opinion, that big part of Oklahoma is a reservation precluding state prosecutions

Proving yet again that he is fully prepared to rule in favor of criminal defendants when he believes he is required to do so by the rule of law, Justice Gorsuch this morning voted with the Supreme Court's more liberal justices to hold in McGirt v. Oklahoma, No. 18–9526 (S. Ct. July 9, 2020) (available here) that a huge part of the state of Oklahoma "remains an Indian reservation for purposes of federal criminal law."  Here is how the opinion of the Court, authored by Justice Gorsuch, gets started:

On the far end of the Trail of Tears was a promise. Forced to leave their ancestral lands in Georgia and Alabama, 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 “[t]he Creek country west of the Mississippi shall be solemnly guarantied to the Creek Indians.” Treaty With the Creeks, Arts. I, XIV, Mar. 24, 1832, 7 Stat. 366, 368 (1832 Treaty).  Both parties settled on boundary lines for a new and “permanent home to the whole Creek nation,” located in what is now Oklahoma. Treaty With the Creeks, preamble, Feb. 14, 1833, 7 Stat. 418 (1833 Treaty). 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.” 1832 Treaty, Art. XIV, 7 Stat. 368.

Today we are asked whether the land these treaties promised remains an Indian reservation for purposes of federal criminal law.  Because Congress has not said otherwise, we hold the government to its word.

The import and impact of this ruling is most clear from the first paragraphs of Chief Justice Roberts' dissent:

In 1997, the State of Oklahoma convicted petitioner Jimcy McGirt of molesting, raping, and forcibly sodomizing a four-year-old girl, his wife’s granddaughter. McGirt was sentenced to 1,000 years plus life in prison.  Today, the Court holds that Oklahoma lacked jurisdiction to prosecute McGirt — on the improbable ground that, unbeknownst to anyone for the past century, a huge swathe of Oklahoma is actually a Creek Indian reservation, on which the State may not prosecute serious crimes committed by Indians like McGirt.  Not only does the Court discover a Creek reservation that spans three million acres and includes most of the city of Tulsa, but the Court’s reasoning portends that there are four more such reservations in Oklahoma.  The rediscovered reservations encompass the entire eastern half of the State — 19 million acres that are home to 1.8 million people, only 10%–15% of whom are Indians.

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.  On top of that, the Court has profoundly destabilized the governance of eastern Oklahoma.  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.

None of this is warranted. What has gone unquestioned for a century remains true today: A huge portion of Oklahoma is not a Creek Indian reservation. Congress disestablished any reservation in a series of statutes leading up to Oklahoma statehood at the turn of the 19th century. The Court reaches the opposite conclusion only by disregarding the “well settled” approach required by our precedents. Nebraska v. Parker, 577 U. S. 481, ___ (2016) (slip op., at 5).