The historical document signed in 1215 is the Magna Carta. Also known as the Great Charter, it was signed by King John of England at Runnymede, following a conflict with his barons over heavy taxation and lack of military support. The Magna Carta established the principle that everyone, including the king, was subject to the law and laid the groundwork for modern democracy and legal rights. It introduced ideas such as due process and the right to a fair trial, significantly influencing subsequent legal systems and constitutional developments throughout history.