power under Osman I and his son Orhan I. The Anatolian beyliks were successively absorbed into the rising Ottoman Empire during the 15th century. It is not well understood how the Osmanlı, or Ottoman Turks, came to dominate their neighbours, as the history of medieval Anatolia is still little known. The Ottomans completed the conquest of the peninsula in 1517 with the taking of Halicarnassus (modern Bodrum) from the Knights of Saint John. Modern times With the acceleration of the decline of the Ottoman Empire in the early 19th century, and as a result of the expansionist policies of the Russian Empire in the Caucasus, many Muslim nations and groups in that region, mainly Circassians, Tatars, Azeris, Lezgis, Chechens and several Turkic groups left their homelands and settled in Anatolia. As the Ottoman Empire further shrank in the Balkan regions and then fragmented during the Balkan Wars, much of the non-Christian populations of its former possessions, mainly Balkan Muslims (Bosnian Muslims, Albanians, Turks, Muslim Bulgarians and Greek Muslims such as the Vallahades from Greek Macedonia), were resettled in various parts of Anatolia, mostly in formerly Christian villages throughout Anatolia. A continuous reverse migration occurred since the early 19th century, when Greeks from Anatolia, Constantinople and Pontus area migrated toward the newly independent Kingdom of Greece, and also towards the United States, the southern part of the Russian Empire, Latin America, and the rest of Europe. Following the Russo-Persian Treaty of Turkmenchay (1828) and the incorporation of Eastern Armenia into the Russian Empire, another migration involved the large Armenian population of Anatolia, which recorded significant migration rates from Western Armenia (Eastern Anatolia) toward the Russian Empire, especially toward its newly established Armenian provinces. Anatolia remained multi-ethnic until the early 20th century (see the rise of nationalism under the Ottoman Empire). During World War I, the Armenian genocide, the Greek genocide (especially in Pontus), and the Assyrian genocide almost entirely removed the ancient indigenous communities of Armenian, Greek, and Assyrian populations in Anatolia and surrounding regions. Following the Greco-Turkish War of 1919–1922, most remaining ethnic Anatolian Greeks were forced out during the 1923 population exchange between Greece and Turkey. Of the remainder, most have left Turkey since then, leaving fewer than 5,000 Greeks in Anatolia today. Geology Anatolia's terrain is structurally complex. A central massif composed of uplifted blocks and downfolded troughs, covered by recent deposits and giving the appearance of a