Kantakouzenos' eldest son, Matthew, also resented being passed over in favour of John V. To placate him, a semi-autonomous appanage covering much of western Thrace was created.

Open warfare broke out in 1352, when John V, supported by Venetian and Serbian troops, launched an attack on Matthew Kantakouzenos.

John Kantakouzenos came to his son's aid with 10,000 Ottoman troops who retook the cities of Thrace, plundering them in the process.

Two years later, their capture of Gallipoli marked the beginning of the Ottoman conquest of the Balkans, culminating a century later in the Fall of Constantinople.

Matthew was captured and held hostage until his ransom was paid by John V Palaiologos, who was now the sole master of a rump state.