Victorious in 1347, John VI Kantakouzenos ruled as co-emperor until his son Matthew Kantakouzenos was attacked by John V in 1352, leading to a second civil war. John V asked the ruler of Serbia, Stefan Dušan for help, and Dušan obliged by sending 4,000 Serbian horsemen to his aid. Matthew Kantakouzenos asked his father for help, and 10,000 Ottoman Turks showed up at Demotika (Didymoteicho) in October 1352 and engaged the forces of John V's Serbian allies in an open field battle that resulted in the destruction of the allies and a victory for the more numerous Turks in the service of the Byzantines. The Ottoman Empire thus acquired its first European territory, at Çimpe and Gallipoli. Able to retake Constantinople in 1354, John V removed and tonsured John VI Kantakouzenos; by 1357, he had deposed Matthew as well, who had been captured by the Serbs and was ransomed to John V.