During the English Civil War (1642-1651), King Charles I was the reigning monarch. His belief in the divine right of kings and his attempts to rule without Parliament led to a deepening conflict between royalists and parliamentarians. The war primarily pitted the King's supporters, known as Cavaliers, against the forces of Parliament, known as Roundheads, led by figures like Oliver Cromwell. The conflict culminated in Charles I's trial and execution in 1649, an unprecedented act that drastically altered the political landscape of England, leading to a brief period of republican rule.