Involvement of foreign rulers in the civil war

The leaders of the two opposing sides in the civil war, involved in the conflict not only the social and religious groups of the time but also foreign states, whose assistance in their struggle they wished to secure. John Kantakouzenos began by seeking the help of the Serbian sovereign, Stefan Dusan, when, after the popular riots of 1341, the aristocracy, of which he himself was a member, was in a state of complete disarray throughout the entire area from Thessalonike to Constantinople. But the alliance between the two men (1342) did not last long and their relations were severed when Thessaly recognised Kantakouzenos as Emperor.

Of course, the willingness of Dusan to become involved in the Byzantine conflict was motivated by the desire to serve his own interests, and on no account was he disposed to help either of the opposing factions to acceed to the throne. He thus abandoned Kantakouzenos and approached the regency in Constantinople, who were also seeking his help Whereupon Kantakouzenos sought and formed an alliance with the emir of Aydin, Umur, (1342) an old friend since the time of Andronikos III. With the help of the Turkish emir he began the conquest of Thrace. In early 1343, Umur's forces conquered Didymoteichon, pillaging the conquered areas as they advanced. Moreover, when in 1344 a Western crusade resulted in the occupation of Smyrna, Umur was forced to turn his attention to his emirate and could no longer offer much assistance to Kantakouzenos, the latter entered into an alliance with the Ottoman Orhan. This was the beginning of the Ottoman invasions of Thrace (1345-6), and the subsequent Turkish conquest of the region may be considered a ultimate consequence of the civil war.

The regency in Constantinople, on the other hand, not only won the support of Dusan, but also that of the tsar of Bulgaria, John Alexander (1342). However, just as the allies of Kantakouzenos had pillaged Thrace, so did the allies of the regency avail themselves of the opportunity to seize certain territories of the Empire.