Constantine was crossing Asia Minor to campaign against the Umayyad Caliphate under Hisham ibn Abd al-Malik on the eastern frontier in June 741 or 742. But during this course Constantine was attacked by the forces of his brother-in-law Artabasdos, the stratēgos of the Armeniac theme. 

Defeated, Constantine sought refuge in Amorion, while the victor advanced on Constantinople and was accepted as emperor. While Constantine now received the support of the Anatolic and Thracesian themes, Artabasdos secured that of the themes of Thrace and Opsikion, in addition to his own Armeniac soldiers.

After the rival emperors had bided their time in military preparations, Artabasdos marched against Constantine, but was defeated in May 743 at Sardis. Three months later Constantine defeated Artabasdos' son Niketas and headed for Constantinople. In early November Constantine was admitted into the capital and immediately turned on his opponents, having them blinded or executed. Perhaps because Artabasdos' usurpation was interconnected with the restoration of veneration of images, Constantine now became perhaps an even more fervent iconoclast than his father.