Following the recapture of Constantinople from the Latins, the Byzantines concentrated their efforts in restoring their hold on Greece. Troops had to be taken from the eastern front in Anatolia and into the Peloponesse, with the disastrous consequence that what land the Nicaean empire held in Anatolia was now open to Ottoman raids. With the raids increasing in frequency and ferocity, Byzantine imperial authority rolled back from Anatolia.
By 1326, lands around Nicaea had fallen into the hands of Osman I. He had also captured the city of Bursa, establishing a capital dangerously close to the Byzantine capital of Constantinople. In 1328, Orkhan, Osman's son, began the siege of Nicaea, which had been in a state of intermittent blockade since 1301. The Ottomans lacked the ability to control access to the town through the lakeside harbour. As a result, the siege dragged on for several years without conclusion.
In 1329, emperor Andronicus III attempted to break the siege. He led a relief force to drive the Ottomans away from both Nicomedia and Nicaea. After some minor successes, however, the force suffered a reverse at Pelekanon and withdrew. When it was clear that no effective Imperial force would be able to restore the frontier and drive off the Ottomans, the city proper fell in 1331.
Nicaea had been in Turkish hands before. It was reconquered by the First Crusade through Byzantine diplomacy in 1097. It had served as the capital of the Greek emperors during the period of the Latin Empire from 1204 to 1261. It was the most important Asian city in the Empire at the time of its fall to Osman. There were no Crusaders left then to retake it. The Ottoman conquests continued apace and Nicomedia fell in 1337.