Byzantine–Ottoman wars
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 of Anatolians and many soon 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.
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