 Political tensions in England began to rise , however , and a potential conflict between King John and an alliance of rebel barons opposed to his rule began to look likely . In early 1215 , King John installed a trusted German mercenary called Ranulph in charge of Berkhamsted Castle and reviewed the defensive arrangements there that April . Civil war broke out later that same year . Initially , the rebels were hampered by a lack of equipment – in particular , siege engines – but in May 1216 the future Louis VIII crossed over the English Channel and joined the rebel cause , bringing with him heavy siege equipment . 
