The establishment of the Byzantine Empire is commonly dated to year 324 when Roman Emperor, Constantine the Great (306-337) moved the imperial capital to Byzantium which came to be known as Constantinople. The Western and Eastern (Byzantine) parts of the Roman Empire were finally divided on the death of Emperor Theodosius I in 395.
In contrary to the Western Roman Empire which was destroyed by the barbarian invasions, the Byzantine Empire managed to repulse the invasions of the Visigoths, Huns, Avars, Slavs, Bulgarians and the Persians which marked the reigns of Arcadius (395-408), Theodosius II (408-450), Marcian (450-457), Leo I (457-474), Leo II (474), Zeno (474-745 and 476-491), Anastasius I (491-518) and Justin I (518-527). Justin’s successor Justinian I (527-565) restored the former power of the Byzantine Empire. Ambitions of Justinian I to restore the territory of the former Roman Empire resulted in successful military campaign against the Vandals in Northern Africa in 533-534, recapture of Italy from the Ostrogoths in so-called Gothic War (535-540 and 542-552) and of southern coast of the Iberian Peninsula from the Visigoths in 552.
Justinian I was triumphal in Western Europe but the Byzantine Empire was greatly weakened by the attacks of the Persians on the east and threatened by the invasions of the Slavs, Bulgarians, Huns and Avars on the north at the beginning of the 6th century. Thus the reigns of Justinian’s successors Justin II (565-574) and Tiberius II Constantine (574-582) were marked by the Persian-Byzantine Wars and Slavic invasions in the north, while much of Italy has been captured by the Lombards. Maurice (582-602) transformed the shattered Byzantine Empire into a well-organized medieval state. He restored the Byzantine authority in Western Europe and North Africa by reorganizing the Byzantine dominions into exarchates ruled by the military governors or exarchs.
Maurice was killed in an army rebellion in 602 and the Byzantine throne was assumed by Phocas (602-610) who served as an officer during Maurice’s Balkan campaigns. The Byzantine Empire reached its lowest point during Phocas’ reign and was greatly weakened by the invasions of the Slavs on the north and of the Persians on the east. The Byzantine throne was in very serious situation assumed by Heraclius (610-641) who deposed Phocas and had him killed. Heraclius decisively defeated the Persians in 629 but he neglected the northern frontiers. Slavic peoples settled in the Balkan by year 615, while Heraclius barely managed to defend Constantinople from the Avars. Heraclius’ reign was also marked by increased Hellenization of Byzantine social, political and cultural life as well as by military reorganization of provinces into Themes.
Heraclius was succeeded by his son Constantine III (641) who died only after four months and was succeeded by his younger half-brother Heraclonas (614). However, rumors that he murdered Constantine III resulted in revolt and his deposition. The new Byzantine Emperor became the son of Constantine III, Constans II (641-668) under the regency of the senators. His early reign was characterized by the invasions of the Arabs who captured Egypt, extended their influence in North Africa and seized the islands of Cyprus, Rhodes, Kos and Crete. The Arabs defeated the Byzantine fleet in the naval Battle at Phoinike (off Lycia) in 655 but they were unable to take advantage of the victory because of the inner conflicts. When the Arab threat on the east ceased Constans II launched a campaign against the Slavs in Macedonia and forced them to recognize the Byzantine rule. Afterwards Constans II concentrated on Italy but the Papacy felt strong enough and refused Monothelitism as a compromise between the Eastern and Western Churches. Meanwhile Constans II became very unpopular and he was assassinated in Syracuse in 668.
The period following the execution of Justinian II in 711 and the accession of Leo III in 717 was marked by a civil war and rapid switches on the Byzantine throne. The period of instability as well as of the Arab threat ended with the accession of Leo III the Isaurian (711-741) whose reign was also notable for a series of edicts against the worship of images (726-729). Leo’s prohibition of veneration of the icons provoked a long struggle over iconoclasm which reached its height under his son and successor Constantine V (741-775). However, many of his rigid decrees against the use of images in worship were abolished by his son and successor Leo IV (775-780), while his wife Irene that acted as regent to Leo’s 10-year-old son and successor Constantine VI (780-797) restored the veneration of icons. Irene summoned the Council of Nicaea in 787 which formally revived the adoration of images. The circles that strongly opposed to the adoration of images supported Constantine VI who wanted to rule as sole emperor. Irene was banished in 790 but she was recalled two years later and granted the title of empress. Shortly after her return Irene organized a conspiracy, overthrown her son and ruled as sole empress from 797 to 802. Irene’s revival of adoration of images improved the relations with the Papacy but the Byzantine influence in Western Europe began to decline.