While the Byzantine Roman and Sassanid Persian empires were both weakened by warfare (602–628), a new power in the form of Islam grew in the Middle East. In a series of rapid Muslim conquests, Arab armies, led by the Caliphs and skilled military commanders such as Khalid ibn al-Walid, swept through most of the Middle East, taking more than half of Byzantine territory and completely engulfing the Persian lands. In Anatolia, they were stopped in the Siege of Constantinople (717–718) by the Byzantines, who were helped by the Bulgarians.