The first period of Ottoman history was characterized by almost continuous territorial expansion, during which Ottoman dominion spread out from a small northwestern Anatolian principality to cover most of southeastern Europe and Anatolia. The political, economic, and social institutions of the classical Islamic empires were amalgamated with those inherited from Byzantium and the great Turkish empires of Central Asia and were reestablished in new forms that were to characterize the area into modern times.
In their initial stages of expansion, the Ottomans were leaders of the Turkish warriors for the faith of Islam, known by the honorific title ghāzī (Arabic: “raider”), who fought against the shrinking Christian Byzantine state. The ancestors of Osman I, the founder of the dynasty, were members of the Kayı tribe who had entered Anatolia along with a mass of Turkmen Oğuz nomads. Those nomads, migrating from Central Asia, established themselves as the Seljuq dynasty in Iran and Mesopotamia in the mid-11th century, overwhelmed Byzantium after the Battle of Manzikert (1071), and occupied eastern and central Anatolia during the 12th century. The ghazis fought against the Byzantines and then the Mongols, who invaded Anatolia following the establishment of the Il-Khanid (Ilhanid) empire in Iran and Mesopotamia in the last half of the 13th century. With the disintegration of Seljuq power and its replacement by Mongol suzerainty, enforced by direct military occupation of much of eastern Anatolia, independent Turkmen principalities—one of which was led by Osman—emerged in the remainder of Anatolia.
In their initial stages of expansion, the Ottomans were leaders of the Turkish warriors for the faith of Islam, known by the honorific title ghāzī (Arabic: “raider”), who fought against the shrinking Christian Byzantine state. The ancestors of Osman I, the founder of the dynasty, were members of the Kayı tribe who had entered Anatolia along with a mass of Turkmen Oğuz nomads. Those nomads, migrating from Central Asia, established themselves as the Seljuq dynasty in Iran and Mesopotamia in the mid-11th century, overwhelmed Byzantium after the Battle of Manzikert (1071), and occupied eastern and central Anatolia during the 12th century. The ghazis fought against the Byzantines and then the Mongols, who invaded Anatolia following the establishment of the Il-Khanid (Ilhanid) empire in Iran and Mesopotamia in the last half of the 13th century. With the disintegration of Seljuq power and its replacement by Mongol suzerainty, enforced by direct military occupation of much of eastern Anatolia, independent Turkmen principalities—one of which was led by Osman—emerged in the remainder of Anatolia.
Following the final Mongol defeat of the Seljuqs in 1293, Osman emerged as prince (bey) of the border principality that took over Byzantine Bithynia in northwestern Anatolia around Bursa, commanding the ghazis against the Byzantines in that area. Hemmed in on the east by the more powerful Turkmen principality of Germiyan, Osman and his immediate successors concentrated their attacks on Byzantine territories bordering the Bosporus and the Sea of Marmara to the west. The Ottomans, left as the major Muslim rivals of Byzantium, attracted masses of nomads and urban unemployed who were roaming through the Middle East searching for means to gain their livelihoods and seeking to fulfill their religious desire to expand the territory of Islam. The Ottomans were able to take advantage of the decay of the Byzantine frontier defense system and the rise of economic, religious, and social discontent in the Byzantine Empire and, beginning under Osman and continuing under his successors Orhan (Orkhan, ruled 1324–60) and Murad I (1360–89), took over Byzantine territories, first in western Anatolia and then in southeastern Europe. It was only under Bayezid I (1389–1402) that the wealth and power gained by that initial expansion were used to assimilate the Anatolian Turkish principalities to the east.
By 1300 Osman ruled an area in Anatolia stretching from Eskişehir (Dorylaeum) to the plains of İznik (Nicaea), having defeated several organized Byzantine efforts to curb his expansion. Byzantine attempts to secure Il-Khanid support against the Ottomans from the east were unsuccessful, and the Byzantine emperor’s use of mercenary troops from western Europe caused more damage to his own territory than to that of the Turks. The Ottomans lacked effective siege equipment, however, and were unable to take the major cities of Bithynia. Nor could they move against their increasingly powerful Turkmen neighbours, the Aydın and Karası dynasties, which had taken over Byzantine territory in southwestern Anatolia. Orhan’s capture of Bursa in 1324 (some sources date the event to 1326) provided the first means for developing the administrative, economic, and military power necessary to make the principality into a real state and to create an army. Orhan began the military policy, expanded by his successors, of employing Christian mercenary troops, thus lessening his dependence on the nomads.
Orhan soon was able to capture the remaining Byzantine towns in northwestern Anatolia: İznik (1331), İzmit (1337), and Üsküdar (1338). He then moved against his major Turkmen neighbours to the south. Taking advantage of internal conflicts, Orhan annexed Karası in 1345 and gained control of the area between the Gulf of Edremit and Kapıdağı (Cyzicus), reaching the Sea of Marmara. He thus put himself in a position to end the lucrative monopoly enjoyed by the city of Aydın, that of providing mercenary troops to competing Byzantine factions in Thrace and at the Byzantine capital, Constantinople (present-day Istanbul). The expansion also enabled the Ottomans to replace Aydın as the principal ally of the Byzantine emperor John VI Cantacuzenus. The consequent entry of Ottoman troops into Europe gave them a direct opportunity to see the possibilities for conquest offered by Byzantine decadence. The collapse of Aydın following the death of its ruler, Umur Bey, left the Ottomans alone as the leaders of the ghazis against the Byzantines. Orhan helped Cantacuzenus take the throne of Byzantium from John V Palaeologus and as a reward secured the right to ravage Thrace and to marry the emperor’s daughter Theodora.
Ottoman raiding parties began to move regularly through Gallipoli into Thrace. Huge quantities of captured booty strengthened Ottoman power and attracted thousands from the uprooted Turkmen masses of Anatolia into Ottoman service. Starting in 1354, Orhan’s son Süleyman transformed Gallipoli, a peninsula on the European side of the Dardanelles, into a permanent base for expansion into Europe and refused to leave, despite the protests of Cantacuzenus and others. From Gallipoli Süleyman’s bands moved up the Maritsa River into southeastern Europe, raiding as far as Adrianople. Cantacuzenus soon fell from power, at least partially because of his cooperation with the Turks, and Europe began to be aware of the extent of the Turkish danger.
Orhan’s son Murad I was the first Ottoman emperor to use Gallipoli for permanent conquests in Europe. Constantinople itself was bypassed, despite the weakness and disorganization of its defenders, because its thick walls and well-placed defenses remained too strong for the nomadic Ottoman army, which continued to lack siege equipment. Murad’s initial conquests extended northward into Thrace, culminating with the capture in 1361 of Adrianople, the second city of the Byzantine Empire. Renamed Edirne, the city became the new Ottoman capital, providing the Ottomans with a centre for the administrative and military control of Thrace. As the main fortress between Constantinople and the Danube River, it controlled the principal invasion road through the Balkan Mountains, assured Ottoman retention of their European conquests, and facilitated further expansion to the north.
Under Sultan Mehmed II (ruled 1451–81) the devşirme increasingly came to dominate and pressed their desire for new conquests in order to take advantage of the European weakness created at Varna. Constantinople became their first objective. To Mehmed and his supporters, the Ottoman dominions in Europe could never reach their full extent or be molded into a real empire as long as their natural administrative and cultural centre remained outside their hands. The grand vizier and other Turkish notables bitterly opposed the attack, ostensibly because it might draw a new Crusade but in fact because of their fear that the capture of the Byzantine capital might bring about the final triumph of the devşirme. Mehmed built Rumeli Fortress on the European side of the Bosporus, from which he conducted the siege (April 6–May 29, 1453) and conquest of Constantinople. The transformation of that city into the Ottoman capital of Istanbul marked an important new stage in Ottoman history. Internally, it meant the end of power and influence for the old Turkish nobility, whose leaders were executed or exiled to Anatolia and whose European properties were confiscated, and the triumph of the devşirme and their supporters in Istanbul and the West. Externally, the conquest made Mehmed II the most famous ruler in the Muslim world, even though the lands of the old caliphate still remained in the hands of the Mamlūks of Egypt and Timur’s successors in Iran. Moreover, the possession of Constantinople stimulated in Mehmed a desire to place under his dominion not merely the Islamic and Turkic worlds but also a re-created Byzantine Empire and, perhaps, the entire world of Christendom.