Nothing is known of his early life. The name of his brother, Apsimar, probably indicates a Germanic origin. In 698, Apsimar was proclaimed emperor by the Byzantine fleet after a failed expedition to recover Carthage. Apsimar set Constantinople under siege and succeeded in entering the city when some officers opened a gate. Deposing the emperor Leontius (r. 695–698), himself a usurper, Apsimar assumed the throne under the old Roman name Tiberius, and set about to gain legitimacy for his regime by securing a military victory against the Empire's major foe, the Umayyad Caliphate.