With Constantinople (Istanbul) as the capital of the Roman Empire in the east, Emperor Constantine I, the Great (306–337), gave the empire a new state center.
To this state belonged inter alia. the Illyrian prefecture, which included Greece and the center of the Balkans. There were three basic elements in the new state: Roman conception of empire, Greek culture and Christian religion. Over time, however, the Latin language (official language until the 7th century) and the Roman idea of empire lost ground, and Byzantium relied more and more on the tradition of ancient Greek culture, so that it basically turned out to be a continuation of Greek culture; therefore is the history of the Byzantine Empire a piece of Greek history par excellence. Although Roman law remained the basis of the Byzantines’ legal awareness, the basis of their intellectual life at all times has been Greek culture, philosophy, poetry, and science. The Greek language was the language of the people in the east of the Roman Empire from the beginning and was used by both the Church Fathers and the Philosophical School of Athens (Academy), which was closed by an edict of 527-565 ruling Emperor Justinian I in 529, when also preferred and supported by the scholars of the University of Constantinople at the time of Emperor Theodosius II (408–450).
Byzantine art, which arose between the 4th and 6th centuries mainly under the influence of the cultures of the Eastern Mediterranean and Near East, also had Greek (Hellenistic) elements (Byzantine culture).
After the partition of the Roman Empire (395), the Visigoths conquered Thrace and Macedonia under the leadership of Alaric I; they advanced into Greece and conquered Thessaly, Central Greece and the Peloponnese, but without destroying Athens. The regent of the Western Empire, Stilicho, forced them to retreat. After the time of the Great Migration, Emperor Justinian I. for stabilization. He defeated Vandals, Eastern and Visigoths, but fought with less success against the Persians, the Slavs and Huns, who conquered part of Greece. In the 7th century the Arabs seized large parts of the empire; what remained was a state with an almost purely Greek population. The Greeks formed the majority in Greece, Asia Minor, the Aegean Islands, Constantinople and the surrounding area, southern Italy and Sicily. During this time, large numbers of Slavs advanced on the Balkan Peninsula as far as the Peloponnese. But the Greek nation was by no means Slavicized during this period and the following. Despite the temporary rule of the Slavs, whose Christianization came from Greek monasteries, Greece was able to preserve its Greek character for the most part uninterrupted. According to physicscat, the heavy Slavic immigration has been declining since the end of the 8th / beginning of the 9th century. The establishment of new Byzantine administrative districts (subjects) in the period from the end of the 7th to the beginning of the 9th century indicates the restoration of Byzantine rule in Greece. The following administrative districts are to be mentioned: Thrace, Aigaion Pelagos (the islands of Archipelagos), Hellas, Macedonia (then Western Thrace), Peloponnese, Kefallinia (Ionian Islands), Thessalonike, Strymon and – continuing the Diocletian division of Epirus – Dyrrhachion and Nicopolis. The island of Crete was occupied by Arabs in 823 / 828–961, who visited the islands and the mainland from here. In 918 the Bulgarian Tsar Simeon I, the Great (893-927), invaded northern Greece and reached the Gulf of Corinth. It was not until the beginning of the 11th century that the Bulgarians were taken over by Emperor Basil II. (976-1025) finally repulsed. The collapse of Byzantine power in Asia Minor and Italy (1071, defeat of Mantzikert against the Seljuks, capture of Bari by the Normans) moved Greece to the center of the empire, but it was now exposed to the incursions of the southern Italian Normans (1084/85 R. Guiscard; in 1147 Roger II of Sicily relocated silk weavers from Thebes and Corinth to Palermo, in 1185 Thessalonike fell).
After the conquest of Constantinople (1204) by the participants in the 4th Crusade, Greece – like the rest of the Byzantine Empire – was the prey of the Venetians and the “Franks”, the crusaders who mostly came from northern France. It disintegrated into small dominions that were only loosely connected to the Latin Empire (1204–61) formed by the Crusaders, to which Thrace, the northwestern part of Asia Minor and several Aegean islands belonged. Margrave Boniface II of Montferrat (* around 1155, † 1207) founded the Kingdom of Thessaloniki (1204-24) in northern Greece, which included the capital of the same name and the neighboring areas of Macedonia and Thessaly. Large principalities emerged in central and southern Greece, the Duchy of Athens (Attica, Boeotia) and the Principality of Achaia. The island of Evia was divided in 1205 under three lords from Lombardy (“Lombard triad”) and came (now mostly called Negroponte) through treaties with them under the influence of Venice, which is also the most important ports and islands of the empire (plus three eighths of Constantinople). In Epirus, on the other hand, a “despotate” emerged under the rule of the Angeloi as the center of Byzantine assertion alongside the empires of the Grand Comnenes in Trebizond and the Laskarids in Nikaia. In 1259 Michael VIII won Palaeologus the cities of Monemvasia, Mistra and Geraki as starting points for the Byzantine recovery of the Peloponnese. From then on, the despotate Morea (or Mistra, after the capital), gradually enlarged at the expense of the Principality of Achaia, remained the only stable power (since 1383 imperial secondary generation of the palaeologists).
Some of the islands came back to the Byzantines as early as 1259, and from them Rhodes to the Order of St. John in 1309. Crete (now named after the capital Candia), a Venetian settlement colony since 1207, finally came under the rule of Venice after an uprising in 1363/66, as did Corfu (1386) and Kefallinia (1500), which had been known since 1194, initially as the Palatinate of Cefallonia, Italian Gentlemen under. North and northwest Greece were lost to the Serbian Tsar Stefan Dušan after the Byzantine reconquest (1246 and 1261) in 1340, then in rapid succession to Italian, Greek, Serbian and Albanian petty princes, and in 1394 and 1430 to the Ottomans. The Duchy of Athens was acquired by the Catalan Company in 1311 conquered and fell in 1388 by purchase to the Florentine banking family Acciaiuoli (in their possession until the conquest of Athens by the Turkish Sultan Mehmed II. 1456). By 1432, the Despotate Morea was able to almost completely eradicate the Frankish rule, to which the ruins of hilltop castles and some Gothic churches testify; in the city of Mistra there was a final cultural flowering (G. G. Plethon), which had an impact on the Renaissance in Italy.