King John of England ("John Lackland") had largely lost his father Henry II's vast empire, raised heavy taxes for wars in France, clashed with the Pope and been excommunicated, then submitted again — in short, his reign was a chain of disasters. In 1215 the English barons openly rebelled, seized London, and prepared an army. Through the mediation of the Archbishop of Canterbury, Stephen Langton, both sides met in a meadow at Runnymede beside the Thames.

On 15 June 1215 John affixed the royal seal to a 63-clause charter: Magna Carta — the "Great Charter". Its original purpose was not a universal declaration of liberty; it regulated the barons' concrete feudal grievances — taxes, military duties, inheritance rules. But certain clauses embedded within it — especially clause 39, which states that "no free man shall be seized or imprisoned, or stripped of his rights or possessions, or outlawed or exiled… except by the lawful judgment of his equals and by the law of the land" — were re-interpreted over the centuries into the foundation of habeas corpus and the right to due process.

The first version of Magna Carta was, in practice, void within months — the Pope annulled it, John went back to war with the barons. But the charter was reaffirmed in the coronation oaths of later Plantagenet kings, repeatedly reissued, and in the 17th century jurists like Edward Coke used it as a tool to constrain royal power. The 1689 English Bill of Rights, the 1776 American Declaration of Independence, the 1789 French Declaration of the Rights of Man, and the 1948 Universal Declaration of Human Rights all refer back to it, directly or indirectly.

Ironically, Magna Carta was written less to limit royal power than to protect the barons' own feudal privileges; its "free man" applied only to the nobility. But the simple act of writing certain principles down and binding the sovereign by his own seal — that gesture became, over the next eight centuries, one of the central symbols of Western constitutional thought.