The Roman Empire in the east, following the fall of the Empire in the west in the 5th century, is conventionally known as the Byzantine Empire (but was simply called “Roman Empire” in its own time) and lasted until 1453. With its capital in Constantinople, its language and literary culture was Greek and its religion was predominantly Eastern Orthodox Christian.
From the 4th century, the Empire’s Balkan territories, including Greece, suffered from the dislocation of the Barbarian Invasions. The raids and devastation of the Goths and Huns in the 4th and 5th centuries and the Slavic invasion of Greece in the 7th century resulted in a dramatic collapse in imperial authority in the Greek peninsula. Following the Slavic invasion, the imperial government retained formal control of only the islands and coastal areas, particularly the densely populated walled cities such as Athens, Corinth and Thessalonica, while some mountainous areas in the interior held out on their own and continued to recognize imperial authority. Outside of these areas, a limited amount of Slavic settlement is generally thought to have occurred, although on a much smaller scale than previously thought.
The Byzantine recovery of lost provinces began toward the end of the 8th century and most of the Greek peninsula came under imperial control again, in stages, during the 9th century. This process was facilitated by a large influx of Greeks from Sicily and Asia Minor to the Greek peninsula, while at the same time many Slavs were captured and re-settled in Asia Minor and the few that remained were assimilated. During the 11th and 12th centuries the return of stability resulted in the Greek peninsula benefiting from strong economic growth – much stronger than that of the Anatolian territories of the Empire.
Following the Fourth Crusade and the fall of Constantinople to the “Latins” in 1204 mainland Greece was split between the Greek Despotate of Epirus (a Byzantine successor state) and French rule (known as the Frankokratia), while some islands came under Venetian rule. The re-establishment of the Byzantine imperial capital in Constantinople in 1261 was accompanied by the empire’s recovery of much of the Greek peninsula, although the Frankish Principality of Achaea in the Peloponnese and the rival Greek Despotate of Epirus in the north both remained important regional powers into the 14th century, while the islands remained largely under Genoese and Venetian control.
In the 14th century, much of the Greek peninsula was lost by the Byzantine Empire at first to the Serbs and then to the Ottomans. By the beginning of the 15th century, the Ottoman advance meant that Byzantine territory in Greece was limited mainly to its then-largest city, Thessaloniki, and the Peloponnese (Despotate of the Morea). After the fall of Constantinople to the Ottomans in 1453, the Morea was the last remnant of the Byzantine Empire to hold out against the Ottomans. However, this, too, fell to the Ottomans in 1460, completing the Ottoman conquest of mainland Greece. With the Turkish conquest, many Byzantine Greek scholars, who up until then were largely responsible for preserving Classical Greek knowledge, fled to the West, taking with them a large body of literature and thereby significantly contributing to the Renaissance.