In 1387, the united forces of the Principality of Serbia and the Kingdom of Bosnia defeated 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. Ivanko did likewise. The Ottomans reacted by sending a 30,000-strong army under the command of the grand vizier Ali Pasha, to the north of the Balkan Mountains, which invaded the Tarnovo Tzardom. After some fighting, Ivan Shishman was forced to sue for peace. The Despotate was also invaded and Varna was besieged. Ivanko was forced to accept vassalage to the Ottoman sultan.