The Ottoman Empire has Turkish origins and Islamic foundations, but from the start it is a heterogeneous mixture of ethnic groups and religious creeds.

Ethnicity is determined solely by religious affiliation.

Non-Muslim peoples, including Greeks, Armenians, and Jews, are recognized as millet and are granted communal autonomy.

Such groups are allowed to operate schools, religious establishments, and courts based on their own customary law.

The day-to-day conduct of Ottoman government and the formulation of policy are in the hands of the divan, a relatively small council of ministers directed by the chief minister, the grand vizier.

The entranceway to the public buildings in which the divan meets—and which in the seventeenth century becomes the residence of the grand vizier—is called the Bab-i Ali (High Gate, or Sublime Porte).

In diplomatic correspondence, the term Porte is synonymous with the Ottoman government, a usage that acknowledges the power wielded by the grand vizier.

The forces of Mehmed the Conqueror in 1453 finally take Constantinople after a lengthy siege.

The great city will again be the center of a Mediterranean empire stretching from Vienna to the Caspian Sea and from the Persian Gulf to the Strait of Gibraltar—but now it will be as a Muslim city in the empire of the Ottomans.

The great Greek state is no more; the fall of Constantinople marks the end of the Eastern Roman Empire.

The Imperial Greeks attempt to retain control of their vestigial territories, but the Turks quickly assume control of the Greek, Latin and Slavic holdings in Anatolia and the Balkans.

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 Ottoman governor of Thessaly, Turahan Bey, breaks through the Hexamilion wall for the fourth time and ravages the Peloponnese peninsula, to prevent the Despotate of the Morea from assisting Constantinople during the final Ottoman siege of the imperial capital, raiding from Corinth through the Argolid and Arcadia to Messenia.

Morea puts up little resistance after Hexamilion, although Turahan's son Ahmed is captured in an ambush at Dervenakia and imprisoned in Mistra.

The Venetians in Constantinople, and a Genoese contingent commanded by Giovanni Giustiniani, wholeheartedly cooperate in the defense of the city, however, when the crisis comes.

Constantine commands an army of only ten thousand men, left as the city's sole defenders after the Turkish navy drives off an assisting Venetian naval fleet and erects a blockade.

Mehmed lays siege to the walls in April 1453.

A chain that the Greeks have thrown across the mouth of the Golden Horn obstructs his ships.

The ships are therefore dragged overland to the harbor from the seaward side, bypassing the defenses.

The Sultan's heavy artillery, fifty-six giant, stone-ball-firing cannons and twelve bombards, continually batters the city's massive land walls, which the defenders are finally unable to block off.

Gunpowder thus becomes recognized as a decisive weapon.

During the siege, the opposing views on the Ottoman side are voiced in two war councils convened at critical moments.

Zaganos vehemently rejects the proposal to raise the siege.

He is given the task of preparing the last great assault.

The commander in chief, Mehmed II himself, on the day of the attack personally directs the operations against the breach opened in the city wall by his cannon.

On May 29, some of his soldiers force their way in.

Giustiniani is mortally wounded.

The emperor Constantine is last seen fighting on foot at one of the gates.

The day after the capture of the city, Candarli is arrested and soon afterwards is executed in Edirne.

He is replaced by Zaganos, who has become Mehmed's father-in-law.

Mehmed had had to consent to a three-day sack of the city, but, before the evening of the first day after its capture, he countermands his order.

Entering the city at the head of a procession, he goes straight to Hagia Sophia and converts it into a mosque, adding four minarets at the perimeter of the structure.

Afterward, he establishes charitable foundations and provides fourteen thousand gold ducats per annum for the upkeep and service of the mosque.

The Turks also convert Saint Savior in the Chora to a mosque, calling it Kariye Djami, and whitewash (and thus protect, perhaps inadvertently) its magnificent mosaics and frescoes.

The eighty thousand-man Turkish army encircling Constantinople, led personally by Mehmed II, inaugurates a siege of the city on April 6, 1453.

Emperor Constantine XI commands an army of only ten thousand men, left as the city’s sole defenders after the Turkish navy drives off an assisting Venetian naval fleet and erects a blockade.

Mehmed uses his fifty-six giant, stone-ball-firing cannons and twelve bombards, all Hungarian-made, to reduce the city’s massive walls, which the imperial forces are finally unable to block off.

The largest of their cannon is the Great Turkish Bombard, which requires an operating crew of two hundred men and seventy oxen, and ten thousand men to transport it.

Gunpowder makes the formerly devastating Greek fire obsolete, and becomes recognized as a decisive weapon.

Many war galleys now mount a large, fixed cannon that fires over the bow.

Such centerline bow guns, often flanked by several smaller guns, will soon become universal.

After a fifty-five-day bombardment, Mehmed enters the city on May 29, 1453; the forty-nine-year-old emperor bravely battles the Turks at one of the city’s gates until he is overwhelmed and killed by the attackers.

Mehmed permits his soldiers three days to plunder the defeated city, after which …

…the victorious Ottoman Turks initiate the transfer of their capital from Edirne to Constantinople and rename it Istanbul.

One of the first acts committed by Mehmet immediately after the conquest of Constantinople is to execute Çandarli Halil Pasha and confiscate his property.

The fact that this execution takes place on June 1, 1453, suggests a design conceived by the sultan for a long time.

Mehmet II has thus ended the period called Çandarli era in the Ottoman Empire, and the later members of family, whose descendants will come down to the present day, will become no more than provincial notables based in İznik, although they are to give yet another, short-term, grand vizier to the Ottoman Empire at the end of the fifteenth century (Halil's son Çandarli Ibrahim Pasha).

Çandarli Halil Pasha is, as such, the first Ottoman grand vizier to be executed.

The fall of Constantinople marks the end of the Greek state and assures the Ottoman conquest of the Orthodox Balkans.

Many Eastern scholars depart for western Europe and bring with them knowledge of Greek manuscripts, and often the manuscripts themselves.

The University of Constantinople, sometimes known as the University of the palace hall of Magnaura, had been founded by Emperor Theodosius II in 425 under the name of Pandidakterion, with thirty-one chairs for law, philosophy, medicine, arithmetic, geometry, astronomy, music, rhetoric and other subjects, fifteen to Latin and sixteen to Greek.

The university has existed until the Ottoman conquest of Constantinople, when a madrasa, a religious school, is established: it is the precursor of Istanbul University.

The restoration of the city now popularly called Istanbul as a worthy capital of a worldwide empire is one of the tasks to which Mehmed II sets his heart.

To encourage the return of the Greeks and the Genoese of Galata (the trading quarter of the city), who have fled, he returns their houses and provides them with guarantees of safety.

In order to repopulate the city, he deports Muslim and Christian groups in Anatolia and the Balkans and forces them to settle in Constantinople.

He restores the Greek Orthodox Patriarchate on January 6, 1454, and establishes a Jewish grand rabbi and an Armenian patriarch in the city.

In addition, he founds, and encourages his viziers to found, a number of Muslim institutions and commercial installations in the main districts of Constantinople.

The victorious Turks had immediately converted the Hagia Sophia for use as a mosque, adding four minarets at the perimeter of the structure.

The Ottomans convert Saint Savior in the Chora to a mosque, calling it Kariye Djami, and whitewash (and thus protect, perhaps inadvertently) its magnificent mosaics and frescoes.

Construction begins on the Ottoman sultan’s Topkapi Palace.

The victorious Ottoman Turks under Sultan Mehmed II move on from their conquest of Constantinople to conquer Albania and Greece, isolating Venetian outposts here.