In 1302 Osman I defeated a Byzantine force near Nicomedia at Bapheus, the first victory in the field by the Ottomans over a Roman army and the beginning of their expansion.
Bapheus is the smallest battle in this milestone and arguably the most consequential. A few thousand men on each side met on a plain outside Nicomedia in the summer of 1302, and the winner's descendants ruled from Budapest to Basra.
The Byzantine Empire had been restored to Constantinople for forty years and was in a bad way: no money, no fleet worth the name, its Anatolian frontier neglected by an emperor whose attention and resources were in Europe.
Osman was the chief of one of the smallest Turkish beyliks, on the Byzantine border in Bithynia. His victory here was the first time the Ottomans beat a Roman army in the field, and its main effect was reputational: it announced that raiding Byzantine territory under Osman was profitable, and men came.
Michael VIII had recovered Constantinople in 1261 and spent the rest of his reign defending it against western attempts to take it back — an expensive success whose cost was Anatolia.
His son Andronikos II inherited the bill. He disbanded the fleet as an economy, which handed the sea to Genoa and Venice, and reduced the frontier troops of the Anatolian marches, who had been the system that kept the beyliks out. The frontier was left to look after itself and it could not.
By 1300 Turkish raiders were operating across Bithynia, the province closest to the capital. Andronikos hired 10,000 Alan mercenaries to deal with the problem, and it is a detachment of these, with local troops, that fought at Bapheus.
The Byzantine commander was the megas hetaireiarches George Mouzalon, with a force of perhaps two thousand men, of whom a large part were the Alan mercenaries. Osman had a similar or slightly larger number of Turkish horsemen, reinforced by fighters drawn in from neighbouring beyliks by the prospect of plunder.
The accounts are brief. George Pachymeres, the only substantial source and a contemporary, describes an Ottoman attack that the Byzantine force could not hold, and the Alans either withdrawing before the fighting or leaving during it — mercenaries who had not been paid and had no stake in Bithynia.
Mouzalon got the remainder into Nicomedia. The field belonged to Osman.
The Ottoman tradition about this battle is a good example of what happens when a dynasty writes its own origins three centuries later. The court manuscripts assign the victory at Koyunhisar — the Turkish name for the battle — to Orhan, Osman's son, who was around twenty in 1302 and did not command.
That is not a copying error but a dynastic preference: Orhan was the ruler who took Bursa and Nicaea and made the beylik a state, and later Ottoman writers built the origin story around the figure they found most useful.
The archive follows Pachymeres, who was writing in Constantinople at the time and had no motive to credit Osman with anything. Where the Greek and the later Ottoman accounts differ about who was in command in 1302, the contemporary source is the one to prefer.
The direct territorial result was modest and slow. The Ottomans did not take a walled city for another quarter century — Bursa fell in 1326, the year Osman died, and Nicaea in 1331 — because they had no siege capability and had to starve towns out.
What Bapheus did was make the countryside impossible. Byzantine Bithynia became a set of isolated towns in a landscape controlled by someone else, and towns in that position eventually surrender.
The reputational effect was larger still. A beylik that beat imperial troops in the open field attracted fighters from every other beylik in Anatolia, and it is that flow of manpower, rather than any territory taken in 1302, that turned the smallest emirate on the map into the state that took Constantinople in 1453.