In 1352, the most destructive, due to conditions, civil war broke out in the Byzantine Empire. The two warring camps fighting to occupy the throne of the broken empire did not hesitate to commit pure treason by calling in help from Serbs and Turks.
The civil war between John Kantakouzenos and John V Paleologus was of crucial importance for the fate of the empire. In the ruins of the state, the two parts of the aristocracy, neither of them possessing their own combat-worthy military forces, called for help from the strongest military states in the region to reinforce them.
The portion of the Paleologues under Andronikos III's widow Anna of Savoy called the Serbs for help against Kantakuzinos, while Kantakouzinos invited, even worse, the Turks of Orhan to help, passing them, for the first time in Europe.
With 10,000 Turks and his headquarters at Didymoteicho, Kantakouzenos occupied or plundered large areas and cities of Thrace. However, the king of Serbia, Stefanos Dusan, to whom Macedonia was granted in return (!) sent 6,000 elite horsemen to reinforce the Paleologians – other sources speak of 4,000.
The Serbian force under Gradislav Borilović met the Turks of Kantakouzen outside the Didymoteichos. In the battle that followed – in which it is a question whether very few Greek warriors took part – the Turks decisively defeated the Serbs.
Ioannis Kantakouzinos speaks of 7,000 Serbs dead, a figure that is excessive by definition, while Nikiforos Grigoras records 4,000 Serbs dead. The number of dead Serbs has little significance, in any case, for the impact that the Turkish victory had on Byzantium.
It was the first victory of the Turks on European soil. It was a victory that filled the Turks with confidence and showed them the utter impotence of Byzantium to resist them... when the time would soon come for them to fight to conquer it for themselves, of course not for the sake of any Cantakuzen or Paleologu.