Complex sentence: When he discovers later that the Khan is planning to invade his homeland of the Song Dynasty, he renounces his loyalty to the Khan and flees from Mongolia to establishes a base of operations within one of Song's strategic strongholds; the city of Xiangyang.
Split into simpler sentences: When he discovers later that the Khan is planning to invade his homeland (the Song Empire), he renounces his loyalty to the Khan and flees from Mongolia. Thereafter, he establishes a base of operations in one of Song's strategic strongholds, the city of Xiangyang.