After centuries of incursion by invaders, the city of Constantinople was captured by the Ottoman Turks. Its fall brought to a close the history of the Roman Empire (the western Empire fell to nomadic invasion in the 5th century; Rome was sacked in 410), although by this time there was little left of its glory and once-expansive territories.

The city was named for the Emperor Constantine the Great (d. 337), who adopted Christianity as the official religion of the Roman Empire. In May of 1453, some 10,000 soldiers under Emperor Constantine XI (r. 1448–53) were overwhelmed by ten times as many well-armed and equipped Ottoman troops. Constantine himself was killed in the siege, fighting for his namesake's legacy.

In 1054, the Christian world was divided in two. Much of western and central Europe recognized the spiritual and political authority of the western, or Roman, Church, led by the Pope in Rome. Territories to the east adhered to Eastern Orthodoxy, with the patriarch in Constantinople at its head. Once Constantinople fell to the Ottoman Turks, it became a center of Islam. Muscovy remained the only independent Eastern Orthodox realm and began assuming the role of defender of the “True Faith,” a role that transcended its borders, drawing it into heightened levels of engagement with the other eastern Christians of Europe and North Africa.