John Cantacuzenus’ relationship with the Turks had been based on personal friendship with their leaders, among them Orhan, to whom he gave his daughter in marriage. But once the Turks had set up a base on European soil and had seen the possibilities of further conquest, such relationships were no longer practicable.
Stefan Dušan, who very nearly realized his ambition to found a new Serbo-Byzantine empire, was the only man who might have prevented the subsequent rapid expansion of the Turks into the Balkans, but he died in 1355 and his empire split up. The new emperor, John V, hoped that the Western world would sense the danger, and in 1355 he addressed an appeal for help to the Pope. The popes were concerned for the fate of the Christian East but guarded in their offers to Constantinople so long as the Byzantine Church remained in schism from Rome.
In 1366 John V visited Hungary to beg for help, but in vain. In the same year his cousin Amadeo, count of Savoy, brought a small force to Constantinople and recaptured Gallipoli from the Turks, who had by then advanced far into Thrace. Amadeo persuaded the Emperor to go to Rome and make his personal submission to the Holy See in 1369. On his way home, John was detained at Venice as an insolvent debtor; during his absence the Turks scored their first victory over the successors of Stefan Dušan on the Marica River near Adrianople in 1371. The whole of Macedonia was open to them. The remaining Serbian princes and the ruler of Bulgaria became their vassals, and in 1373 the Emperor was forced to do the same.
The Byzantine Empire in 1355.Encyclopædia Britannica, Inc
Byzantium became a vassal state of the Turks, pledged to pay tribute and to provide military assistance to the Ottoman sultan. The possession of Constantinople thereafter was disputed by the Emperor’s sons and grandsons in a series of revolutions, which were encouraged and sometimes instigated by the Turks, the Genoese, or the Venetians. John V’s son Andronicus IV, aided by the Genoese and the sultan Murad I, mastered the city for three years (1376–79).
He rewarded the Turks by giving back Gallipoli to them, and Murad made his first European capital at Adrianople. The Venetians helped John V to regain his throne in 1379, and the empire was once again divided into appendages under his sons. Only his second son, Manuel, showed any independence of action.
For nearly five years, from 1382 to 1387, Manuel reigned as emperor at Thessalonica and laboured to make it a rallying point for resistance against the encroaching Turks. But the city fell to Murad’s army in April 1387. When the Turks then drove deeper into Macedonia, the Serbs again organized a counteroffensive but were overwhelmed at Kossovo in 1389.