Completion of Strasbourg Cathedral's North Tower (1439)…

Ottoman power revives by the 1420s to the extent that fresh campaigns are undertaken in Greece.

Aside from scattered outposts in Greece, all that remains of the Empire is its capital, Constantinople.

Cut off by land since 1365, the city, despite long periods of truce with the Turks, is supplied and reinforced by Venetian traders who control  its commerce by sea.

Mehmed II (r. 1444-46, 1451-81), on becoming sultan in 1444, immediately sets out to conquer the city.

The military campaigning season of 1453 commences with the fifty-day siege of Constantinople, during which Mehmed II brings warships overland on greased runners into the Bosporus inlet known as the Golden Horn to bypass the chain barrage and fortresses that had blocked the entrance to Constantinople's harbor.

The Turks fight their way through the gates of the city on May 29 and bring the siege to a successful conclusion.

As an isolated military action, the taking of Constantinople does not have a critical effect on European security, but to the Ottoman Dynasty the capture of the imperial capital is of supreme symbolic importance.

Mehmet II regards himself as the direct successor to the Roman emperors.

He makes Constantinople the imperial capital, as it had been under his Greek and Latin predecessors, and sets about rebuilding the city.

The cathedral of Hagia Sophia is converted to a mosque, and Constantinople—which the Turks call Istanbul (from the Greek phrase eis tin polin, "to the city")—replaces Baghdad as the center of Sunni Islam.

The city also remains the ecclesiastical center of the Greek Orthodox Church, of which Mehmet II proclaims himself the protector and for which he appoints a new patriarch after the custom of the former emperors.

The sultan, who acts in political, military, judicial, social, and religious capacities, under a variety of titles, is at the apex of the hierarchical Ottoman system.

He is theoretically responsible only to God and God's law—the Islamic seriat (in Arabic, sharia), of which he is the chief executor.

All offices are filled by his authority, and every law is issued by him in the form of a firman (decree).

He is supreme military commander and has official title to all land.

Selim I, during the early sixteenth-century Ottoman expansion in Arabia, also adopts the title of caliph, thus indicating that he is the universal Muslim ruler.

The sultan's powers, theocratic and absolute in theory and in principle, are limited in practice.

The attitudes of important members of the dynasty, the bureaucratic and military establishments, and religious leaders have to be considered.

Three characteristics are necessary for acceptance into the ruling class: Islamic faith, loyalty to the sultan, and compliance with the standards of behavior of the Ottoman court.

The last qualification effectively excludes the majority of common Turks, whose language and manners are very different from those of the Ottomans.

The language of the court and government is Ottoman Turkish, a highly formalized hybrid language that includes Persian and Arabic loanwords.

In time, Greeks, Armenians, and Jews are also employed in state service, usually in diplomatic, technical, or commercial capacities.

Selim I (r. 1512-20) extends Ottoman sovereignty southward, conquering Syria, Palestine, and Egypt.

He also gains recognition as guardian of the holy cities of Mecca and Medina.