In January 1649 a reigning English king walked to a scaffold outside his own palace and was publicly beheaded by his subjects. Charles I had waged war against Parliament since 1642 over whether the crown ruled by divine right or was bound by law, with bitter religious conflict between Puritans and the king's church running through everything. Parliament's New Model Army defeated him, tried him as a tyrant, and executed him. The republic that followed collapsed into Cromwellian dictatorship, and the monarchy returned in 1660 — but the precedent that a king must answer to the law did not.
No reigning monarch in Europe had ever been tried and executed by his own subjects. In January 1649, England did exactly that — convicting Charles I as a tyrant and beheading him in daylight, before a silent crowd, in the name of an idea that outlived the short-lived republic it created: that even a king must answer to the law.