On 26 September 1371, the Ottomans defeated a large Christian army led by the Serbian brothers Vukašin Mrnjavčević and Jovan Uglješa in the Battle of Chernomen. They immediately turned on Bulgaria and conquered northern Thrace, the Rhodopes, Kostenets, Ihtiman, and Samokov, effectively limiting the authority of Ivan Shishman in the lands to the north of the Balkan mountains and the Valley of Sofia. Unable to resist, the Bulgarian monarch was forced to become an Ottoman vassal, and in return he recovered some of the lost towns and secured ten years of uneasy peace.