On 25 May 717, Leo III the Isaurian was crowned Emperor of Byzantium. During the summer of the same year the Arabs, led by Maslama ibn Abd al-Malik, crossed the Dardanelles and besieged Constantinople with a large army and navy.

Leo III made a plea to Tervel for help, relying on the treaty of 716, and Tervel agreed. The first clash between the Bulgars and the Arabs ended with a Bulgar victory. During the very first stages of the siege the Bulgars appeared in the Muslim rear and large part of their army was destroyed and the rest were trapped. The Arabs built two trenches around their camp facing the Bulgarian army and the walls of the city. They persisted with the siege despite the severe winter with 100 days of snowfall. In the spring, the Byzantine navy destroyed the Arab fleets that had arrived with new provisions and equipment, while a Byzantine army defeated Arab reinforcements in Bithynia. Finally, in early summer the Arabs engaged the Bulgars in battle but suffered a crushing defeat. According to Theophanes the Confessor, the Bulgars slaughtered some 22,000 Arabs in the battle. Shortly after, the Arabs raised the siege. Most historians primarily attribute the Byzantine–Bulgarian victory with stopping the Arab offensives against Europe.