context: John initially adopted a defensive posture similar to that of 1199: avoiding open battle and carefully defending his key castles. John's operations became more chaotic as the campaign progressed, and Philip began to make steady progress in the east. John became aware in July that Arthur's forces were threatening his mother, Eleanor, at Mirebeau Castle. Accompanied by William de Roches, his seneschal in Anjou, he swung his mercenary army rapidly south to protect her. His forces caught Arthur by surprise and captured the entire rebel leadership at the battle of Mirebeau. With his southern flank weakening, Philip was forced to withdraw in the east and turn south himself to contain John's army.
question: ఫిలిప్పు ఎందుకు తూర్పువైపుకు వెళ్ళవలసి వచ్చింది?
answers: southern flank weakening