For High Treason
Charles I famously engaged in a struggle for power with the Parliament of England. He was responsible for the English Civil War against parliament.
Charles was defeated in the first Civil War (1642 - 1645), after which Parliament expected him to accept demands for a constitutional monarchy. He instead remained defiant by attempting to forge an alliance with Scotland and escaping to the Isle of Wight. This provoked a second Civil War (1648 - 1649) and a second defeat for Charles, who was subsequently captured, tried, convicted, and executed for high treason.