The Byzantine–Ottoman wars were a series of decisive conflicts between the Byzantine Greeks and Ottoman Turks and their allies that led to the final destruction of the Byzantine Empire and the rise of the Ottoman Empire. The Byzantines, already having been in a weak state even before the dismemberment of their Empire following the Fourth Crusade, failed to recover fully under the rule of the Palaiologos dynasty. Thus, the Byzantines faced increasingly disastrous defeats at the hands of the Ottomans. Ultimately, they lost Constantinople in 1453, formally ending the conflicts (however, several Byzantine holdouts lasted until 1479).
Taking advantage of a weakening Byzantine Empire, the Seljuk Sultanate of Rum began seizing territory in western Anatolia, until the Nicaean Empire was able to repel the Seljuk Turks from the remaining territories still under Byzantine rule. Constantinople was retaken from the Latin Empire in 1261 by the Nicaean Empire, but the position of the Byzantine Empire in Europe remained uncertain due to the presence of the rival powers in Epirus, Serbia and Bulgaria. This, combined with the declining power of the Sultanate of Rum (Byzantium's chief rival in Asia Minor) led to the removal of troops from Anatolia to maintain Byzantium's grip on Thrace.
The decay of the Sultanate of Rum brought unexpected instability to the Anatolian frontier, as warriors known as ghazis began setting up fiefdoms at the expense of the Byzantine Empire. While many Turkish beys participated in the conquest of Byzantine and Seljuk territory, the territories under the control of one such bey, Osman I, posed the greatest threat to Constantinople. Within 90 years of Osman I's establishment of the Ottoman beylik, the Byzantines lost all of their Anatolian territory and by 1400, Byzantine Thrace was also lost to the Ottomans. Timur's invasion of 1402, and the subsequent Ottoman Interregnum allowed a ruined Constantinople to stave off defeat until it finally fell in 1453. After having taken the city, Ottoman supremacy in the eastern Mediterranean was largely secured. This helped lead to the end of the Middle Ages and the beginning of the Age of Discovery.
Rise of the Ottomans: 1265–1328
Following the Byzantine reconquest of Constantinople in 1261, the Byzantine Empire was left in an isolated position. There was plenty of talk among the remaining Latin duchies of the Greek mainland and other regions of retaking Constantinople for the Latin Empire whilst to the north another significant threat came from Serbian expansion into the Balkans by king Stefan Uroš I.
What was once a strong frontier under the Komnenian dynasty at the Danube river now threatened Constantinople itself. To solve these problems, Michael VIII began consolidating his rule; he had the younger co-emperor John IV blinded, which resulted in much resentment. To counter this, the Byzantine Emperor installed a new Patriarch of Constantinople, Germanus III, ordering him to lift an excommunication that had been placed against him by the former Patriarch Arsenios Autoreianos and to submit to the authority of Rome in order to alleviate the Latin threat. As the Byzantine Empire continued the conquest of Latin territory, the Turks under Osman I began their raids into Byzantine Anatolia; Söğüt and Eskişehir were taken in 1265 and 1289 respectively. Michael Palaiologos was unable to deal with these early setbacks due to the need to transfer troops to the West. In 1282, Michael Palaiologos died and his son Andronikos II took power. The death of the old Byzantine Emperor came as a relief for the society at large; his policy of Latin appeasement to the Church in Rome, heavy taxation and military expenditure placed a severe burden on the people. As the Ottoman Turks began taking land from the Empire, they were seen as liberators by some Anatolians and a significant minority converted to Islam, undermining the Byzantine's Orthodox power base.
Andronikos' rule was marked by incompetence and short-sighted decisions that in the long run would damage the Byzantine Empire beyond repair. He began to debase the Byzantine hyperpyron, resulting in a reduction of the value of the Byzantine economy; taxes were decreased for the Powerful, i.e. landed aristocracy and instead placed upon the Knight-class Pronoia. To popularize his rule he repudiated the union of the Orthodox and Catholic Churches decreed by the Second Council of Lyon in 1274, thereby further increasing hostilities between the Latins and the Byzantines. Andronikos II took a deep interest in preserving the Anatolian lands of Byzantium and ordered construction of forts in Asia Minor and vigorous training of the army. The Byzantine Emperor ordered that his court be moved to Anatolia to oversee the campaigns there and instructed his General Alexios Philanthropenos to push back the Turks. Early successes were rendered useless when Alexios staged an unsuccessful coup, leading to his blinding and the end of his campaigns. This allowed the Ottomans to lay siege to Nicaea in 1301. A further defeat on Andronikos' son Michael IX and the Byzantine general George Mouzalon occurred at Magnesia and Bapheus in 1302. Despite this, Andronikos tried once more to strike a decisive blow back at the Turks, this time hiring Catalan mercenaries. Under the guidance of Michael IX and the leadership of Roger de Flor, the 6,500-strong Catalan Company in the spring and summer of 1303 managed to drive back the Turks. The mercenaries' onslaught drove the Turks back from Philadelphia to Cyzicus, in the process causing great destruction to the Anatolian landscape. Once again these gains were thwarted by internal matters. Roger de Flor was assassinated and, in revenge, his company began pillaging the Anatolian countryside. When they finally left in 1307 to attack Byzantine Thrace, the locals welcomed the Ottomans who once again began blockading key fortresses in Asia Minor. The Ottomans were able to build on their military success due to the numerous divisions amongst their opponents. Many of the peasant classes in Anatolia saw the Ottomans as better masters.
After these defeats, Andronikos was in no position to send substantial forces. In 1320, Andronikos II's grandson, Andronikos III, was disinherited following the death of his father, Michael IX, the Emperor's son and heir apparent. The following year, Andronikos III retaliated by marching on Constantinople and was given Thrace as an appanage. He kept on pressing for his inheritance and, in 1322, was made co-emperor. This culminated in the Byzantine civil war of 1321–1328, in which Serbia backed Andronikos II and the Bulgarians backed his grandson. Eventually Andronikos III emerged triumphant on May 23, 1328. As Andronikos III consolidated his hold on Byzantium, the Ottomans succeeded in taking Bursa from the Byzantines in 1326.
Byzantium counter: 1328–1341
The fate of Nicaea was sealed when the Byzantine relief army was defeated at Pelekanos on 10 June 1329. In 1331, Nicaea surrendered, resulting in a massive blow due to its strategic position relative to Constantinople.
Once again the Byzantines' military power was depleted and Andronikos III were forced into diplomacy as his grandfather was before him; in return for the safety of the remaining Byzantine settlements in Asia Minor, tribute would be paid to the Ottomans. Unfortunately for the Byzantine Empire, this did not stop the Ottomans from laying siege to Nicomedia in 1333; the city finally fell in 1337. In 1338, the Ottoman leader Orhan Gazi took Skoutarion, giving the Ottomans a base within sight of Constantinople for the first time.
Despite these setbacks, Andronikos III was able to score a few successes against his opponents in mainland Greece and Aegean seas; Thessaly along with Epirus were retaken in 1333 and 1337 respectively. In 1329, the Byzantines recaptured Chios, Phocaea in 1334, and, in 1335, secured Lesbos. Nonetheless, these isolated Islands were isolated exceptions to the general trend of increasing Ottoman conquests. Furthermore, none of the Islands were a part of the Ottoman domain; their capture demonstrates the potential that the Byzantines had at the time of Andronikos III. Byzantine military ability would be further weakened by Serbian expansions into recent acquisitions by Andronikos III (Epirus) and finally by a devastating civil war that would subjugate the Byzantine Empire as a vassal to the Ottomans.
Balkan invasion and civil war: 1341–1371
Andronikos III died in 1341 leaving his 10-year-old son John V to rule. A regency was set up with John Cantacuzenus, the young Emperor's mother, Anna of Savoy, and the Patriarch John XIV Kalekas. Rivalries between Kalekas and Cantacuzenus led to a destructive civil war, in which Cantacuzenus emerged triumphant at Constantinople in February 1347. During this time plague, earthquakes and Ottoman raiding continued until only Philadelphia remained in Byzantine hands and only so by payment of a tribute. Throughout the civil war the Byzantines on both sides employed Turks and Serbs with mercenaries pillaging at will, leaving much of Macedonia in ruin and in the hands of the newly created Serbian Empire. Following this victory, Cantacuzenus ruled as co-emperor with John V.
This dual rule eventually failed and the two waged a new civil war further diminishing what was left of Byzantium's integrity in the eyes of her troublesome neighbors. John VI Cantacuzenus emerged triumphant once again and replaced the now exiled John V Palaiologos with his son Matthew Cantacuzenus as junior co-emperor. However, the Turks, under Osman I's son, Orhan I, now came into play by occupying Tzimpe in 1352, before capturing the fort of Kallipolis (Gallipoli) in 1354 and gaining access to the European mainland after captured Chalcedon, the Asiatic suburb of Constantinople in 1353. The arrival of the seemingly unbeatable Ottoman soldiers surrounding Constantinople caused a panic in Constantinople, capitalized by John V who, with the assistance of the Genoese, staged a coup and ousted John VI Cantacuzenus in November 1354. As a result, John VI would later become a monk.
The civil war did not end there; Matthew Cantacuzenus now obtained troops from Orhan and began a bid for taking Constantinople. His capture in 1356 ended his dreams of becoming Emperor and with it came an ephemeral defeat for the Ottomans who had favored the overthrow of John V, Following the end of the civil conflict came a small lull in fighting between the expanding Ottomans and Byzantines. In 1361 Didymoteichon fell to the Turks. Orhan's successor, Murad I was more concerned with his Anatolian positions. However, just like Alp Arslan of the Seljuk Turks, Murad I left the taking of Byzantine territory to his vassals with Philippopolis falling after major campaigning between 1363–64 and Adrianople succumbing to the Ottomans in 1369.
The Byzantine Empire was in no position to launch any decent counter-attack or defense of these lands; by now the Ottomans had become supremely powerful. Murad I crushed an army of Serbians on 26 September 1371 at the Battle of Maritsa leading to the end of Serbian power. The Ottomans were now poised to conquer Constantinople. In an attempt to stave off defeat, John V appealed to the Pope for support offering submission to Rome in return for military support. Despite publicly confessing the Roman Catholic Faith in St. Peter's Basilica, John V received no help. John V therefore was forced to turn to reason with his enemies, the Ottomans. Murad I and John V then came to an agreement whereby Byzantium would provide regular tribute in troops and money in exchange for security.
Crusade from the West planned against Ottoman Empire managed to temporarily recapture Gallipoli in 1366. However, civil war between John V and his son, Andronikos IV allowed Turks to reoccupy it due to the latter's handed it over as payment for Murad's support against the former.
Byzantine civil war and vassalage: 1371–1394
By now the Ottomans had essentially won the war; Byzantium was reduced to a few settlements other than Constantinople and was forced to recognize its vassal status to the Ottoman Sultan. This vassalage continued until 1394. However, whilst Constantinople had been neutralized, the surrounding Christian powers were still a threat to the Ottomans and Asia Minor was not under complete Ottoman control. The Ottomans continued their thrust into the Balkans, proving to be great conquerors in Europe as they were in Anatolia; in 1385 Sofia was captured from the Bulgarians and Niš was taken the following year. Meanwhile, various smaller states were subjugated as vassals, including the Serbs following the Battle of Kosovo in 1389, much of Bulgaria was taken in 1393 by Bayezid I. By 1396 the Bulgarians had been entirely subjugated when Vidin fell.
Ottoman advances into the Balkans were aided by further Byzantine civil conflict – this time between John V and his eldest son Andronikos IV. With Ottoman aid from Murad I, John V was able to blind Andronikos IV and his son John VII Palaiologos in September 1373. Andronikos escaped with his son and secured Murad's aid by promising a higher tribute than John V's. The civil strife continued as late as September 1390 though potential for conflict continued until 1408. John V eventually forgave Andronikos IV and his son in 1381, angering his second son and heir apparent, Manuel II Palaiologos . He seized Thessalonika, alarming the Ottoman Sultan in liberating parts of Greece from Ottoman rule.
The death of Andronikos IV in 1385 and the capitulation of Thessalonika in 1387 to Hayreddin Pasha encouraged Manuel II Palaiologos to seek the forgiveness of the Sultan and John V. His increasingly close relationship with John V angered John VII who saw his right as the heir threatened. John VII launched a coup against John V but despite Ottoman and Genoese aid his reign lasted mere five months before he was toppled by Manuel II and his father.