Complex sentence: During Arghun's reign, the Egyptian Mamluks continuously reinforced their power in Syria, and the Mamluk Sultan Qalawun re-captured Crusader territories, some of which, such as Tripoli, had been vassal states of the Il Khans.
Split into simpler sentences: During Arghun's reign, the Egyptian Mamluks were continuously reinforcing their power in Syria. The Mamluk Sultan Qalawun recaptured Crusader territories, some of which, such as Tripoli, had been vassal states of the Il Khans.