You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
John,_King_of_England

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:
Why was Philip forced to withdraw in the east?

southern flank weakening