In 717 the Umayyad Caliphate launched the siege of Constantinople (717–718) which lasted for one year. However, the combination of Leo III the Isaurian's military genius, the Byzantines' use of Greek Fire, a cold winter in 717–718, and Byzantine diplomacy with the Khan Tervel of Bulgaria resulted in a Byzantine victory. After Leo III turned back the Muslim assault in 718, he addressed himself to the task of reorganising and consolidating the "themes" in Asia Minor. In 740 a major Byzantine victory took place at the Battle of Akroinon where the Byzantines destroyed the Umayyad army once again.