The Ottoman Sultanate (1299-1922 as an empire; 1922-1924 as caliphate only), also referred to as the Ottoman Empire, written in Turkish as Osmanlı Devleti, was a Turkic imperial state that was conceived by and named after Osman (l. 1258-1326), an Anatolian chieftain. At its peak in the 16th and 17th centuries, the empire controlled vast stretches including Anatolia, southwestern Europe, mainland Greece, the Balkans, parts of northern Iraq, Azerbaijan, Syria, Palestine, parts of the Arabian Peninsula, Egypt, and parts of the North African strip, in addition to the major Mediterranean islands of Rhodes, Cyprus, and Crete. Renowned the strongest military superpower of its time, the empire stagnated and faced prolonged decline from the late 16th century CE onwards until it was replaced by the modern Republic of Turkey after the First World War (1914-1918). In the 11th century, the Seljuk Turks, a people from the Asian steppe who had accepted the Sunni version of Islam, swept over Persia and neighboring eastern territories and then advanced westwards towards Anatolia. There, they dealt the imperial forces of the Byzantine Empire (330-1453) a devastating defeat near Manzikert in 1071, and henceforth several Turkic tribes settled the region. By the end of the 13th century, the various Anatolian beyliks (petty kingdoms) were virtually independent but feuding amongst each other. Osman (r. 1299-1326), the bey (chieftain) of Bithynia, a region situated westwards, near the Sea of Marmara, initiated a war with the bordering Byzantine realm, expanding his domains at their expense and laying siege to Prusa (Bursa) which fell after his death in 1326. AFTER A CIVIL WAR, MEHMED I EMERGED AS THE UNRIVALED RULER OF THE UNIFIED OTTOMAN REALM, & HE IS OFTEN DUBBED AS THE SECOND FOUNDER OF THE EMPIRE. Osman's successors swept over the Byzantine holdings in Anatolia and Europe, even taking over the Balkans by the close of the 14th century. The Europeans made vehement attempts to fight off the Ottomans but they failed, most notably at the pivotal battles of Kosovo (1389) and Nicopolis (1396). The Turks met their match, not from the west but the east, when they clashed with the rival Timurid forces (over a territorial conflict in Anatolia) under the Turko-Mongol leader Timur (aka Tamerlane, r. 1370-1405) near Ankara in 1402. The Ottomans were defeated, and Sultan Bayezid I (r. 1389-1402) was captured. However, the western powers failed to exploit this opportunity to its fullest, and after a civil war, otherwise known as the Ottoman Interregnum (1402-1413), Mehmed I (r. 1413-1421), a son of Bayezid, emerged victorious as the unrivaled ruler of the unified Ottoman realm, and for this, he is often dubbed as the second founder of the empire. Having restored the empire's borders as they were before the Battle of Ankara, the Ottomans appeared before the legendary Theodosian Walls of Constantinople, the last bastion of the Byzantine Empire, in 1453, under Mehmed II the Conqueror (r. 1451-1481, a grandson of Mehmed I).