Greece's medieval history is directed by the Byzantine Empire which turned around Christianity, Greek Language and Civilization, and Roman law. It was a powerful force in the Mediterranean basin for centuries, involving in trade, politics, and the spread of Christianity. The empire collaborated with Rome during the Crusades against the Muslims. However, during the 13th century, the Crusaders turned on the Byzantine Empire itself and sacked Constantinople. With a weakened Byzantine Empire, Frankish and Latin invaders arrived and inhabited several places of Greece. Over the following centuries, the Byzantine Empire began to regain strength and regain lost territory, but received a final blow in the 15th century when a growing Ottoman Turkish Empire to the east conquered Constantinople.
Constantinople was renamed Istanbul. The Ottoman state was a theocracy and its political system was based on hierarchy with at the top, the Sultan, which has the absolute divine rights. The Ottomans separated the non-Muslim community into millets : Armenian, Catholic, Jewish, and Orthodox. The Ottomans gave to the millets a huge part of autonomy. The ruler of each millet was the religious leader who was responsible for his subject s obedience to the Sultan. The head of the Orthodox millet was the ecumenical patriarch of Constantinople. The patriarch had much authority and assumed an important role to the progress of the Greek Orthodox society. The Ottoman state had, for several reasons, a much decentralised administration. They designed local military leaders, and later the Empire was separated into regions that were governed by Pashas". Official contact was limited to tax collection and military conscription. Orthodox priests and Christian primates collected taxes and maintained order but on the other hand kept the Greek language and traditions alive and allowed to keep the national identity.
The Ottoman system categorized the non-Muslim inhabitants by imposing special taxes like the cizye , a head tax and tax for freedom. According to Greece s History, during the Ottoman s domination, Greeks speakers families moved in many countries: in Romania, Russia, the Hasburg Empire. They monopolized the trade between the Ottoman Empire and the outside world. These diaspora communities also assumed an important role in the development of a Greek identity. They were influenced by all the modern currents, including the ideology of revolution. Many prosperous "diaspora" people became rich and helped the Greeks by founding schools and other public institutions. Movements for independence by minorities began to multiply in the introduction of the 19th century.