The Battle of Blair Mountain is the closest the U.S. came to a civil war in the 20th century. In late August 1921, an armed standoff between union miners (above: random union miner) and company strike busters in West Virginia escalated into a full-on battle that lasted over a week, involved machine guns and aerial bombardment, and killed up to 100 people (via History). When the dust settled, the union miners were the ones on the losing side, and West Virginia came down on them, hard.
Over 1,000 miners surrendered to the army, and the state selected a few to absolutely throw the book at. Two were tried with treason, and one guy, a miner named Walter Allen, became the first person since the Civil War era to be convicted of it. According to When Miners March, a history of the uprising, Allen was sentenced to ten years but was allowed to lodge an appeal. He duly did, was granted bail, paid it, and then simply vanished. It's been speculated he no longer trusted the state to deliver justice and so went into hiding. As a result, his conviction was never overturned. His appeal is still on file with the state Supreme Court, presumably on the off chance that he stumbles out of a time machine one day and decides to resume his case.
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