The first Slavic people to fall under Byzantine influence were the Bulgarians. From the time these barbarians crossed the Danube in the late seventh century, they engaged in intermittent warfare against the Byzantine Empire. At the same time, a Slavic people called the Moravians had established a state of their own. Their rulers associated Christianity with their powerful neighbors, the Germans, and feared both German and papal encroachment.
Byzantium and Islam
Perhaps the major Byzantine cultural achievement was the transmission of their civilization to the Slays. Much as Rome Christianized large groups of “barbarians” in western Europe, so Constantinople, the new Rome, Christianized in eastern Europe.
Although intrigue and the violent overthrow of sovereigns remained a feature of Byzantine politics, the people developed a deep loyalty to the new ruling house that was established in 867 by the Armenian Basil I (r. 867-886) and called the Macedonian dynasty because of his birth there. As political disintegration began to weaken the opposing Muslim world, the Byzantines counterattacked in the tenth century. They captured Crete in 961 and Antioch and much of northern Syria in 962.
In 717-718 Leo III, who had come to the throne as a successful general, defeated the Arabs who were besieging Constantinople. Thereafter the Byzantine struggle against the Muslims gradually became stabilized along a fixed frontier in Asia Minor.
The emperors immediately following Constantine were Arians until Theodosius I (r. 379-395), who in 381 proclaimed orthodox Nicene Athanasian Christianity to be the sole permitted state religion. All those who did not accept the Nicene Creed were to be driven from the cities of the Empire. The Empire, East and West, was united under Theodosius, but his sons Arcadius (r. 395-408) and Honorius (r. 395-423) divided it, with Arcadius ruling at Constantinople.
Despite their efforts, the emperors at Constantinople could not reconquer the West and thus reconstitute the Roman Empire of Augustus. Indeed, theological controversy, reflecting internal political strain, and combined with Persian and Arab aggression, cost the Empire Syria and Egypt. The internal structure was modified to meet the new situation.