In the Byzantine civil war which began in 1352, John Palaiologos obtained the help of Serbia, while John Kantakouzenos sought help from Orhan I, the Ottoman bey. Kantakouzenos marched into Thrace to rescue his son, Matthew, who was attacked by Palaiologos shortly after being given this appanage and then refusing to recognize John Palaiologos as heir to the throne.
The Ottoman troops retook some cities that had surrendered to John Palaiologos, and Kantakouzenos allowed the troops to plunder the cities, including Adrianople, thus it seemed that Kantakouzenos was defeating John Palaiologos, who now retreated to Serbia. Emperor Stefan Dušan sent Palaiologos a cavalry force of 4,000 or 6,000 under the command of Gradislav Borilović while Orhan I provided Kantakouzenos 10,000 horsemen.Also bulgarian tsar Ivan Alexander send unknown number of troops to support Palaiologos and Dušan. The two armies met at an open-field battle near Demotika (modern Didymoteicho) in October 1352, which would decide the fate of the Byzantine Empire, without the direct involvement of the Byzantines. The more numerous Ottomans defeated the Serbs, and Kantakouzenos retained the power, while Palaiologos fled to Venetian Tenedos. According to Kantakouzenos about 7,000 Serbs fell at the battle (deemed exaggerated), while Nikephoros Gregoras (1295–1360) gave the number as 4,000. The battle was the first major battle of the Ottomans on European soil, and it made Stefan Dušan realize the major threat of the Ottomans to Eastern Europe.