In 1369, the Ottoman Turks under Murad I conquered Adrianople (in 1363) and made it the effective capital of their expanding state. At the same time, they also captured the Bulgarian cities of Philippopolis and Boruj (Stara Zagora). As Bulgaria and the Serbian princes in Macedonia prepared for united action against the Turks, Ivan Alexander died on 17 February 1371. He was succeeded by his sons Ivan Sracimir in Vidin and Ivan Šišman in Tǎrnovo, while the rulers of Dobruja and Wallachia achieved further independence.

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 Maritsa. 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.