Leo entered Constantinople on 25 March 717 and forced the abdication of Theodosios III, becoming emperor as Leo III. The new emperor was immediately forced to attend to the Second Arab siege of Constantinople, which commenced in August of the same year. The Arabs were Umayyad forces sent by Caliph Sulayman ibn Abd al-Malik and serving under his brother Maslama ibn Abd al-Malik. They had taken advantage of the civil discord in the Byzantine Empire to bring a force of 80,000 to 150,000 men and a massive fleet to the Bosphorus.