The Umayyads, emboldened by Leontius' perceived weakness, invaded the Exarchate of Africa in 696, capturing Carthage in 697. Leontius sent the patrikios John to retake the city. John was able to seize Carthage after a surprise attack on its harbor. However, Umayyad reinforcements soon retook the city, forcing John to retreat to Crete and regroup. A group of officers, fearing the Emperor's punishment for their failure, revolted and proclaimed Apsimar, a droungarios (mid-level commander) of the Cibyrrhaeots, emperor.

Apsimar took the regnal name Tiberius, gathered a fleet and allied himself with the Green faction, before sailing for Constantinople, which was enduring the bubonic plague. After several months of siege, the city surrendered to Tiberius, in 698. The Chronicon Altinate gives the date February 15. Tiberius captured Leontius, and had his nose slit before imprisoning him in the Monastery of Dalmatou.