With the conquest of Constantinople in 1453 by Mehmed the Conqueror, the fall of the Eastern Roman or Byzantine Empire was a fact. The Ottoman Empire entered a new phase. Military expeditions led to the annexation of Serbia, the Peloponnese, Bosnia, Herzegovina, Euboea and northern Albania. The southern part of Crimea and the Crimean Khanate became a Ottoman client state. This conquered area with Anatolia would remain the core area of the Ottoman Empire until the 19th century.

Mehmed allowed the Orthodox Church to maintain its autonomy and land in exchange for accepting Ottoman authority. The majority of the Orthodox population accepted Ottoman rule as preferable to Venetian rule. Albanian resistance was a major obstacle for Ottoman expansion on the Italian half-island. During the reign of Sultan Mehmed II (between 1451 to 1481), the Ottoman state was transformed into a centralized empire. The Ottomans did not use the term ‘empire’, but called it the Sublime State.

In the 15th and 16th century, the Ottoman Empire entered a period of expansion. A line of committed and effective sultans ruled the empire. Sultan Selim I, for example, expanded the Empire’s eastern and southern border by defeating Safavid Iran in the Battle of Chaldiran. He established Ottoman rule in Egypt by defeating and annexing the Mamluk Sultanate of Egypt, which resulted the Ottomans created a naval presence on the Red Sea. After this expansion, competition began with Portuguese Empire and the Ottoman Empire become the dominant power in the region.

It also flourished economically due to its control of the major overland routes between Europe and Asia. The new capital Istanbul (the renamed Constantinople) was repopulated with Armenian, Jewish and Greek populations. When the population of Istanbul was about 40.000 before the conquest, by 1480 this was already 75.000 (almost the double). By the middle of the 16th century the population expanded to 400.000 people. Centralization peaked under the reign of Sultan Suleiman the Magnificent (1520 to 1566). After his historic victory in the Battle of Mohács, he established Ottoman rule in the territory of the present-day (2021) Hungary, with exception of the western part. He tried to conquer Vienna but was repulsed. Transylvania, Wallachia and Moldavia became tributary principalities of the Ottoman Empire. He did not stop there, he also took Bagdad from the Persians, gained control of Mesopotamia ad naval access to the Persian Gulf. The Caucasus had a partitioning with the Ottomans. Venice surrendered too. France and the Ottoman Empire allied against mutual opposition Habsburg rule. Suleiman I died of natural causes in his tent during the Siege of Szigetvár in 1566.

By the end of Suleiman’s reign, the Ottoman Empire became a dominant naval force controlling much of the Mediterranean Sea. They were also a major part of the European political sphere.