Osman I
Battle of Bapheus

Soon after Osman secured his independence and established control over all fortresses he conquered, he sent messages to all remaining Byzantine tekfurs in Anatolia asking them to choose between accepting Islam, Ottoman sovereignty and paying jizyah, or war. Some of these tekfurs ended up accepting Islam, including Osman's old friend Köse Mihal, who became the Turkic leader's companion, and would play a considerable part in the upcoming expansions of the Osmanic beylik. His descendants became known in Ottoman history as Mihaloğulları (children of Michael, plural of Mihaloğlu). Other governors acknowledged Osman's sovereignty, while the rest kept their loyalty to the Byzantine Emperor. Thus, Osman started harassing their fortresses such as Bursa and Nicaea which was besieged in 1301. The Ottoman raids also threatened the port city of Nicomedia with famine, as the Ottomans roamed the countryside prohibiting peasants from harvesting wheat. This provoked Bursa's tekfur among others to unite their efforts to eliminate this new emerging Islamic power.

In the spring of 1302, Emperor Michael IX launched a campaign that reached south to Magnesia. The Ottomans, awed by his large army, avoided an open battle. The Emperor sought to confront The Ottomans, but he was dissuaded by his generals. Encouraged by that, The Ottomans resumed their raids, virtually isolating the Emperor at Magnesia. Soon, the imperial army started dissolving without engaging in a single battle, that is because the local troops left to defend their homes which were continuously raided by the Ottomans, and the Alan mercenaries left as well, aiming to rejoin with their families in Thrace. The Byzantine emperor was forced to withdraw by the sea, followed by a wave of refugees. To counter the threat to Nicomedia, Michael's father, Andronikos II, sent a Byzantine force of some 2,000 men (half of whom were recently hired Alan mercenaries), under the megas hetaireiarches, Giorgios Mouzalon, to cross the Bosporus and relieve the city.

The Byzantine response was a warning for the Islamic border villages and towns. However, when the locals noticed Osman's leadership and military strength, as well as his devotion to Islam, they rallied to support and stand with him to consolidate a new Islamic state that would unite them and form an impenetrable wall against the Byzantines. Several Byzantine deserters joined Osman as well, some of which were liberated prisoners of war who chose to align with him, reportedly due to his good treatment during their custody. Many Islamic warrior brotherhoods also joined the Ottomans. For example, the Gazi Rûm's (Raiders of the Romans), they were stationed on the borders of the Byzantine Empire and repelled its attacks on Muslim lands since the Abbasid era, gaining great experiences and knowledge in Byzantine strategies and tactics. Another example is the Ḥajjian Rûm's (pilgrims of the Romans), a brotherhood of Muslim clergy concerned with teaching local villagers and recent converts the basics and different aspects of Islam, and had a side objective of assisting the Mujahideen in combat.

The Byzantine and Ottoman armies eventually met on 27 July 1302 at the plain of Bapheus located between Nicomedia and Nicaea. The Ottoman army consisted of light cavalry under Osman himself, and they numbered around 5,000, while the Byzantines numbered around 2,000 men. The Muslim cavalry charged toward the Byzantines fast, whose Alan contingent did not participate in the battle. As a result of the attack, the Byzantine line was broken, forcing Giorgios Mouzalon to withdraw into Nicomedia under the cover of the Alan force.

Bapheus was the first major victory for the nascent Osmanic Beylik, and of major significance for its future expansion: the Byzantines effectively lost control of the countryside of Bithynia, withdrawing to their forts, which became isolated and fell one by one eventually. The Byzantine defeat also sparked a mass exodus of the Christian population from the area into the European parts of the empire, further altering the region's demographic balance. Coupled with the defeat at Magnesia, the Ottomans were able to reach the coasts of the Aegean Sea, threatening Byzantium with a final loss for their territory in Asia Minor. According to Halil İnalcık, the battle allowed the Ottomans to achieve the characteristics and qualities of a true state.
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