Only a few months after the ascension of Ivan Shishman to the throne, on 26 September 1371, the Ottoman Turks defeated a large Christian army led by the Serbian brothers Vukašin Mrnjavčević and Jovan Uglješa in the Battle of Chernomen. Although Uglješa had tried to create a broad coalition that would include Bulgaria, Ivan Shishman, who had to strengthen his own authority, did not join. After their victory at Chernomen, the Turks immediately turned on Bulgaria. Ottoman sultan Murad I forced Ivan Shishman to retreat to the north of the Balkan Mountains and conquered northern Thrace, the Rhodopes, Kostenets, Ihtiman and Samokov. Unable to resist the attacks, Ivan Shishman had to negotiate with the Ottomans in 1373. He was forced to become an Ottoman vassal and to allow his sister Kera Tamara, who was known for her beauty, to become a wife of Murad I. Under this agreement, Bulgaria regained some of the conquered territories such as Ihtiman and Samokov, and began nearly ten years of uneasy peace with the Turks. Despite the vassalage and the peace treaty, Ottoman raids were renewed in the beginning of the 1380s and culminated in 1385 with the fall of Sofia, the last stronghold of Ivan Shishman to the south of the Balkan Mountains.