King Charles I was executed in 1649 primarily because he was found guilty of high treason by the English Parliament. The execution followed a series of political and military conflicts during the English Civil War, where Charles fought to maintain his authority against Parliament. Parliament accused him of trying to uphold tyrannical and unlimited power to rule according to his will, overthrowing the rights and liberties of the people. His trial declared him a traitor for causing the war and for attempting to rule without Parliament’s consent. Despite his refusal to acknowledge the court's authority, he was sentenced to death and executed by beheading outside the Banqueting House in Whitehall, London. This marked the first time a reigning English monarch was publicly tried and executed, symbolizing a shift in power towards parliamentary governance over the monarchy.