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The case centered on Allen Ryan Alleyne, who was convicted of using a firearm for a crime after he robbed a store manager who was driving cash to a bank at the end of his shift - a crime that carried a 5-year minimum sentence.
The jury did not find he had "brandished" the weapon, which carried a 7-year mandatory minimum sentence. A judge said Alleyne could still get the 7-year minimum, finding a jury didn't need to find him guilty of "brandishing" the gun and that a judge could make that finding. A federal appeals court affirmed that ruling.
In overturning that decision, Thomas wrote in the majority opinion that any finding made by a judge that increases a defendant's mandatory minimum sentence violates the Sixth Amendment's right to a trial by a jury of one's peers.
"The judge, rather than the jury, found brandishing, thus violating petitioner's Sixth Amendment rights," Thomas wrote.
Thomas's opinion was joined by Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor, and Elena Kagan. The conservatives, and swing voter Anthony Kennedy, dissented.
While it's unusual for Thomas to side with the liberals, he does have a history of favoring defendants' right to a jury trial. In 1998, Thomas ruled that people or companies sued for copyright infringement have a right to a jury trial when the plaintiff wants statutory damages.
It's also not entirely surprising that a conservative justice would side with liberals in a criminal case. Unlike cases involving like affirmative action and gay marriage, criminal cases can be less politicized for the court.
Just a couple of weeks ago, Thomas's conservative colleague Antonin Scalia sided with liberals himself when he ruled that cops shouldn't be able to collect your DNA without a warrant.