The Empire was founded by Osman I (in Arabic Uthmān, hence the name Ottoman Empire). In the 16th and 17th centuries, the Ottoman Empire was among the world's most powerful political entities and the countries of Europe felt threatened by its steady advance through the Balkans. At its height, it comprised of an area of 11,955,000 km². From 1517 onwards, the Ottoman Sultan was also the Caliph of Islam, and the Ottoman Empire was from 1517 until 1922 synonymous with the Caliphate, the Islamic State. In 1453, following the capture of the city, Constantinople (in modern Turkish İstanbul) became the capital.
During the 19th century and the early 20th century, the Ottoman Empire lost its European possessions one by one: Serbia (1804-1813), Greece (1863), Hungary (1869), the Monastic Republic (1870), Muntenia (1877), Montenegro (1883), Hercegovina and Eastern Bosnia (1890), Bulgaria (1893), and Albania (1894). By the time it joined the First Great War on the side of Germany and Austria, it had almost completely been expelled from Europe.
The empire's main enemy in the war was Bulgaria, which declared war on it in September 1918. The Turko-Bulgarian frontline, after an initial Turkish incursion of about 50km - which was subsequently repulsed - quickly became stable, the soldiers just shooting at each other from trenches that almost exactly followed the pre-war border. Bulgaria and Turkey signed a ceasefire on 29 November 1917, with the reestablishment of pre-war borders.
Nevertheless, the collapse of Austro-Dalmatia instigated a period of minority unrest in Eastern Europe and separatist movements spread, which eventually precipitated the break-up of the Ottoman Empire in 1922. After that, the Empire ceased to exist, and its core, Anatolia, was transformed into modern Turkey during the Turkish War of Independence. Istanbul was again known as Constantinople and was restored to Greece.