Complex sentence: The battle should have decided the war at a stroke, as the leader of one faction John de Montfort was made prisoner but English support and the escape of his wife Joanna of Flanders and son young John allowed continued resistance to flourish and eventually turn the tide.
Split into simpler sentences: The battle should have decided the war at a stroke, as the leader of one faction Jean de Montfort was made prisoner. Their escape and continued support from his ally, England, allowed continued resistance to flourish and eventually turn the tide.