Which English king signed the Magna Carta in 1215?

John, and he did not sign it - he sealed it, since medieval documents were authenticated with wax rather than signatures and it is unclear whether he could write. He also had it annulled by the Pope within ten weeks, on the grounds that it had been extracted under duress, which it plainly had.

It was a peace treaty between a king and rebellious barons about baronial grievances - scutage, wardship, forest law, fish weirs - and it failed immediately, producing civil war. It was reissued repeatedly by later kings in altered form, and only three of its clauses remain law in England today.

Its modern meaning is largely a seventeenth-century construction. Lawyers opposing the Stuarts, particularly Edward Coke, reached back and reinterpreted it as a foundational guarantee of liberty against arbitrary royal power - and that reading crossed the Atlantic and shaped American constitutional thinking. What survives is the durable clause: that no free man shall be imprisoned or dispossessed except by lawful judgement of his peers or by the law of the land. Free man excluded most of the population at the time, and the principle outgrew its own drafting.