During the 3rd century AD, Greece was invaded by various tribes from the Balkans and Eastern Europe that were fighting the Roman army. The Pax Romana was endangered. Deep social and economic problems rose all over the Empire and taxes were increased to expand or reorganize the army.
In the meantime, Christianity had been recognized as a religion in the Empire. In 305 AD, Constantine became the Emperor of Rome. In 324 AD, Emperor Constantine I transferred the capital of the Empire from Rome to Byzantium which took the name of Constantinople, meaning the town of Constantine.
In 364 AD, the Empire was officially split and the Roman Empire was divided into two parts: the Roman Empire at the west and the Byzantine Empire at the east. The Roman Empire started to decline after the invasion of barbarian troops from the North.
The strategic location of Constantinople, between the Black Sea and the Aegean, allowed controlling trade and developing the economy. Even if Constantine gave legitimacy to Christianity, paganism continued to exist until it gradually disappeared. During the 6th century, Emperor Justinian expanded the empire's territory by conquering the southern Levant, northern Africa, and Italy. He organized a centralized bureaucracy and a new fiscal system.
However, the Empire was engaged in several wars that left it very vulnerable. Serious threats would frequently come both from the East and West. During the 9th century, the Byzantine Empire was ruled by a Macedonian dynasty which conquered new territories in the Middle East and opened new trade lines. These military successes improved the economic status of the Empire.
These periods of prosperity were followed by a decline after the 11th century. In 1208 AD, the Knights of Saint John seized Constantinople and caused damages to a large part of the town. After that, the Venetians conquered the largest part of the Greek mainland and the islands. These territories were lost by the Venetians to the Ottomans in the 14th century. Constantinople finally fell to the Ottomans in 1453 BC, marking the end of the Byzantine period.