Justinian's wars brought the empire to the verge of bankruptcy and left it in a vulnerable military position. Threats from both East and West plunged the empire into a spiral of decline that lasted for nearly 300 years. The first menace from the East came from the Persian Sassanid Empire. Sassanid forces took Palestine, Syria, and Egypt, and even threatened Constantinople at one point. A more serious threat soon developed with the advent of Islamic expansionism. Exploding out of the Arabian Peninsula, Muslim forces swept northward and westward, taking Egypt, Syria, Iraq, Iran, and Afghanistan. Portions of Asia Minor were wrested from the Byzantine Empire, and twice between A.D. 668 and 725 Constantinople was nearly overrun by Muslim forces.

The other major threat to the empire came from the West. During the late sixth and seventh centuries, Slavic peoples began to invade the Balkan Peninsula. Major cities such as Athens, Thebes, and Thessaloniki were safe behind defensible walls. Much of the indigenous population of the Balkans, Greeks included, fled, especially to Calabria at the southern tip of Italy, or relocated their settlements to higher, more secure regions of the Balkans. Under these conditions, urban centers no longer were the basis of Byzantine society in the Balkans.

But the Slavic arrivals were unable to preserve their own distinct cultural identities; very soon their hellenization process began. Greek remained the mother tongue of the region, and Christianity remained the dominant faith. Although the Slavic invasions and Islamic conquests of the seventh and eighth centuries shrank the Byzantine state, it survived as a recognizable entity grounded more firmly than ever in the Balkans and Asia Minor.

Byzantine Empire