In 1387, the united forces of the Principality of Serbia and the Kingdom of Bosnia managed to defeat the Ottomans in the Battle of Pločnik. Encouraged by the Christian success, Ivan Shishman immediately invalidated his vassalage to Murad I and refused to send troops in his support in 1388. The Ottomans reacted by sending a 30,000-strong army, under the command of the Grand Vizier Çandarlızade Ali Pasha, to the north of the Balkan Mountains. The Ottoman troops seized the fortresses of Shumen, Madara, Venchan and Ovech. Ivan Shishman left Tarnovo and headed to Nikopol, where he was besieged and forced to ask for peace. The Ottomans requested that he reconfirm his vassalage in addition to surrendering Silistra, at the time the most populous Bulgarian city along the Danube. However, Ivan Shishman, reassured by his neighbours that he would receive support and the preparations of Serbia for war, not only refused to let the Ottomans in the city, but also strengthened its walls. Ali Pasha crossed the Balkan Mountains for a second time to consecutively capture Shumen, Cherven, Svishtov and once again besiege Ivan Shishman in Nikopol. Surprised by the swift Ottoman response and having not received the promised assistance, the Bulgarian emperor had to ask for peace. His pleas were accepted, but the terms were harsher than the original: not only Silistra was to be surrendered, but Ottoman garrisons were to be stationed in other Bulgarian cities, most notably Shumen and Ovech.