English barons forced their king to sign away some of his own power in 1215

Facing rebellion from barons furious over high taxes and arbitrary justice, King John of England sealed Magna Carta at Runnymede in 1215, agreeing that even the monarch was bound by law. Most of its specific clauses are long obsolete, but the principle that rulers aren't above the law became a touchstone for constitutional government centuries later.

— English constitutional history, 1215, Magna Carta — sealed at Runnymede, June 1215