Initial expansion
Orhan invaded Byzantine territories in northwest Anatolia. First, in 1321, Orhan captured Mudanya, the port linking the city of Bursa to the Sea of Marmara coast. He then sent a column under Konur Alp towards the west Black Sea coast; another column under Akça Koca to capture Kocaeli, and finally a column under Abdurrahman Ghazi to capture the southeast coast of the Sea of Marmara. Then, he captured Bursa by persuading its Byzantine commander to defect. As Evrenos Bey, he became a commander of light cavalry, and he and his sons and grandsons led Ottoman troops in the Balkans. Once Bursa was captured, Orhan sent cavalry towards the Bosphorus, capturing Byzantine towns on the Marmara coast. There were even sightings of Ottoman light cavalry along the Bosphorus coast, and Ottoman forces laid siege to Nicaea (second only to Constantinople in the Byzantine Empire).
In 1329, Byzantine Emperor Andronikos III led a mercenary army to relieve Nicaea and regain Kocaeli. In the ensuing battle of Pelekanon (near Nicomedia, Bithynia; present day Maltepe, Turkey), on 10 or 15 June, Orhan's disciplined troops routed the Byzantines. Thereafter Andronikos abandoned the idea of getting Kocaeli back and never again conducted a field battle against Ottoman forces.
Nicaea surrendered to Orhan after a three-year siege that concluded in 1331. Orhan captured Nicomedia in 1337. Orhan gave the command of Nicomedia to his eldest son, Suleyman Pasha, who had directed the operations of the siege.
In 1338, Orhan captured Scutari; most of northwest Anatolia was in Ottoman hands. The Byzantines still controlled the coastal strip from Şile on the Black Sea to Scutari and the city of Amastris (now Amasra) in Paphlagonia, but these were so scattered and isolated as to be no threat to the Ottomans.
In 1345, there was a change of strategy. Instead of aiming to gain land from non-Muslims, Orhan took over a Turkish principality, Karesi (present Balıkesir and surroundings). According to the Islamic philosophy of war, areas under Islamic rule were to be abodes of peace and the other areas abodes of war. Conducting a war in abodes of war was considered a good deed. Karesi principality was a state governed by a Turkish emir and its main inhabitants were Turkish so it was an abode of peace. The Ottomans had to have special justification for conquering fellow Muslim Turkish principalities.
In the case of Karesi, the ruler had died and had left two sons whose claims to the post of emir were equally valid. So there was a fight between the armed supporters of the two claimant princes. Orhan's pretext for invasion was that he was acting as a bringer of peace. In the end of the invasion by Ottoman troops the two brothers were pushed to the castle of their capital city of Pergamum (now Bergama). One was killed and the other was captured. The territories around Pergamum and Palaeocastro (Balıkesir) were annexed to Orhan's domains. This conquest was particularly important since it brought Orhan's territories to Çanakkale, the Anatolian side of the Dardanelles Straits.
With the conquest of Karesi, nearly the whole of northwestern Anatolia was included in the Ottoman Beylik, and the four cities of Bursa, Nicomedia, Nicaea, and Pergamum had become strongholds of its power. At this stage of his conquests, Orhan's Ottoman Principality had four provinces:
Original land grant area of Söğüt and Eskişehir;
Hüdavendigar (Domain of the Sultan) area of Bursa and İznik;
Koca Eli peninsular area around İzmit;
former principality of Karesi around Balıkesir and Bergama.
