Unable to resist the attacks , Ivan Shishman had to negotiate with the Ottomans in 1373 .
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 Ottoman in 1373 .
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 .