Byzantine–Ottoman wars
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 partitioning of their Empire following the 4th 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 the situation, the Seljuk Sultanate of Rum began seizing territory in western Anatolia, until the Nicaean Empire was able to repulse the Seljuk Turks from the remaining territories still under Byzantine rule. Eventually Constantinople was re-taken from the Latin Empire in 1261 by the Nicaean Empire. The position of the Byzantine Empire in Europe remained uncertain due to the presence of the rivals 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 nobles 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 Nicaea and 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. The Crusade of Nicopolis in 1396, Timur's invasion of 1402, and the Crusade of Varna in 1444 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.
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