The Uprising against Thomas the Slav

After Emperor Leo V the Armenian died in 820, Michael II the Stammerer ascended the throne but faced opposition from part of the military aristocracy and former supporters of Leo V, led by Thomas the Slav.
Khan Omurtag intervened on Michael II's side against Thomas's uprising. It began in Asia Minor, but the army crossed into the Balkans. Many Slavs from Macedonia and Thrace joined it, and it besieged Constantinople. Omurtag's intervention was probably in fulfilment of the treaty concluded between Bulgaria and Byzantium in 815, which provided thirty years of peace and included clauses for military assistance. He advanced against the insurgents with a large army, and they lifted the siege.
 

The insurgents suffered a heavy defeat in battle with the Bulgarian army. Thomas the Slav sought refuge in Arcadiopolis, but its inhabitants captured him and handed him over to the emperor.