During the 16th and 17th centuries, at the height of its power under the reign of Suleiman the Magnificent, the Ottoman Empire was a multinational, multilingual empire controlling most of Southeast Europe, parts of Central Europe, Western Asia, parts of Eastern Europe and the Caucasus, North Africa and the Horn of Africa. At the beginning of the 17th century, the empire contained 32 provinces and numerous vassal states. Some of these were later absorbed into the Ottoman Empire, while others were granted various types of autonomy during the course of centuries.
In Western Europe, the two names "Ottoman Empire" and "Turkey" were often used interchangeably, with "Turkey" being increasingly favoured both in formal and informal situations. This dichotomy was officially ended in 1920–23, when the newly established Ankara-based Turkish government chose Turkey as the sole official name. Most scholarly historians avoid the terms "Turkey", "Turks", and "Turkish" when referring to the Ottomans, due to the empire's multinational character.
The Balkan territories lost by the Ottomans after 1402, including Thessaloniki, Macedonia and Kosovo, were later recovered by Murad II between the 1430s and 1450s. On 10 November 1444, Murad repelled the Crusade of Varna by defeating the Hungarian, Polish, and Wallachian armies under Wladyslaw III of Poland (also King of Hungary) and John Hunyadi at the Battle of Varna, although Albanians under Skanderbeg continued to resist. Four years later, John Hunyadi prepared another army of Hungarian and Wallachian forces to attack the Turks, but was again defeated at the Second Battle of Kosovo in 1448.
France and the Ottoman Empire, united by mutual opposition to Habsburg rule, became strong allies. The French conquests of Nice (1543) and Corsica (1553) occurred as a joint venture between the forces of the French king Francis I and Suleiman, and were commanded by the Ottoman admirals Barbarossa Hayreddin Pasha and Turgut Reis. A month before the siege of Nice, France supported the Ottomans with an artillery unit during the 1543 Ottoman conquest of Esztergom in northern Hungary. After further advances by the Turks, the Habsburg ruler Ferdinand officially recognized Ottoman ascendancy in Hungary in 1547.
Before the reforms of the 19th and 20th centuries, the state organisation of the Ottoman Empire was a system with two main dimensions, the military administration and the civil administration. The Sultan was the highest position in the system. The civil system was based on local administrative units based on the region's characteristics. The state had control over the clergy. Certain pre-Islamic Turkish traditions that had survived the adoption of administrative and legal practices from Islamic Iran remained important in Ottoman administrative circles. According to Ottoman understanding, the state's primary responsibility was to defend and extend the land of the Muslims and to ensure security and harmony within its borders in the overarching context of orthodox Islamic practice and dynastic sovereignty.