Thessaly remained part of the East Roman "Byzantine" Empire after the collapse of Roman power in the west, and subsequently suffered many invasions, such as by the Slavic tribe of the Belegezites in the 7th century AD. The Avars had arrived in Europe in the late 550s. They asserted their authority over many Slavs, who were divided into numerous petty tribes. Many Slavs were galvanized into an effective infantry force, by the Avars. In the 7th century the Avar-Slav alliance began to raid the Byzantine Empire, laying siege to Thessalonica and even the imperial capital Constantinople itself.