The conquest movements in the Marmara Region during the reigns of Osman Gazi and Orhan Gazi

Emerging as a frontier principality (uç beyliği) in northwestern Anatolia along the Byzantine borderlands, particularly in close proximity to the region of Bithynia, the Ottomans gradually consolidated their power and evolved into a world empire that would endure for nearly six centuries. The conquest campaigns conducted in the Marmara region—where the firm foundations of the Ottoman polity were laid—were shaped to a great extent by the leadership of Osman Gazi and Orhan Gazi. During the reign of Osman Gazi, Ottoman expansion was largely facilitated by the weakening political and military authority of Byzantium in the region. In this period, major urban centers such as Nicaea (İznik) and Bursa were subjected to prolonged sieges, while the capture of strategically significant locations including Karacahisar, Bilecik, and İnegöl contributed to the steady enlargement of Ottoman territorial control. Osman Gazi's policy of conquest was not limited to military success alone but was closely tied to a strategy of permanent settlement and colonization, which enabled the Ottomans to establish a lasting presence in the Marmara basin. Following the transfer of authority from Osman Gazi to his son Orhan Gazi, the conquest of Bursa in 1326 marked a critical turning point in Ottoman history. With Bursa's designation as the capital, the Marmara region emerged as the political, administrative, and economic center of the Ottoman state. Subsequent conquests—the capture of Nicaea in 1331 and Nicomedia (İzmit) in 1337—effectively severed Byzantium's remaining connections in Anatolia along the Marmara corridor. These developments allowed the Ottomans to secure dominance around the Sea of Marmara and to create favorable conditions for expansion into Rumelia. Under the leadership of Orhan Gazi's eldest son, Süleyman Paşa, the Ottoman crossing into Rumelia—an event extensively recorded in contemporary chronicles—initiated systematic gaza activities in the Balkans. This phase constituted one of the most decisive steps in laying the groundwork for the Ottomans' transformation from a regional frontier polity into an imperial power.