Complex sentence: When military defeat cleared the way for Henry III to take the throne, de Quincy went on crusade, perhaps in fulfillment of an earlier vow, and in 1219 he left to join the Fifth Crusade, then besieging Damietta.
Split into simpler sentences: When military defeat cleared the way for Henry III to take the throne, de Quincy went on crusade, perhaps in fulfillment of an earlier vow. In 1219 he left to join the Fifth Crusade, then besieging Damietta.