Turkey (Turkish: Türkiye), known officially as the Republic of Turkey is a Eurasian country that stretches across the Anatolian peninsula in Western Asia and Thrace in the Balkan region of southeastern Europe. Turkey is bordered by eight countries: Bulgaria to the northwest; Greece to the west; Georgia to the northeast; Armenia, Azerbaijan (the exclave of Nakhchivan) and Iran to the east; and Iraq and Syria to the southeast. The Mediterranean Sea and Cyprus are to the south; the Aegean Sea to the west; and the Black Sea is to the north. Separating Anatolia and Thrace are the Sea of Marmara and the Turkish Straits (the Bosphorus and the Dardanelles), which are commonly reckoned to delineate the boundary between Europe and Asia,[4] thereby making Turkey a country of significant geostrategic importance. The predominant religion in Turkey is Islam and its official language is Turkish. The Turks began migrating into the area now called Turkey in the eleventh century. The process was greatly accelerated by the Seljuk victory over the Byzantine Empire at the Battle of Manzikert. Several small beyliks and the Seljuk Sultanate of Rûm ruled Anatolia until the Mongol Empire's invasion. Starting from the thirteenth century, the Ottoman beylik united Anatolia and created an empire encompassing much of Southeastern Europe, Western Asia and North Africa. After the Ottoman Empire collapsed following its defeat in World War I, parts of it were occupied by the victorious Allies. A cadre of young military officers, led by Mustafa Kemal Atatürk, organized a successful resistance to the Allies; in 1923, they would establish the modern Republic of Turkey with Atatürk as its first president. Turkey is a democratic, secular, unitary, constitutional republic, with an ancient and historical cultural heritage. Turkey has become increasingly integrated with the West through membership in organizations such as the Council of Europe, NATO, OECD, OSCE and the G-20 major economies. Turkey began full membership negotiations with the European Union in 2005, having been an associate member of the European Economic Community since 1963 and having reached a customs union agreement in 1995. Turkey has also fostered close cultural, political, economic and industrial relations with the Eastern world, particularly with the Middle East and the Turkic states of Central Asia, through membership in organizations such as the Organisation of the Islamic Conference and Economic Cooperation Organization. Turkey is classified as a developed country[5] by the CIA and as a regional power[6][7] by political scientists and economists worldwide.