In late 1565 and early 1566, tensions between the Holy Roman Emperor Maximilian II and Sultan Suleiman grew. Maximilian wanted the cities previously taken by Telli Hasan Pasha restored to him. When negotiations failed, Maximilian declared war and Grand Vizier Sokollu Mehmed Pasha ordered his nephew, Sokollu Mustafa Bey of Bosnia, to advance against Maximilian. Mustafa managed to capture the cities of Krupa and Dvor na Uni. The Sultan immediately declared war against the Holy Roman Empire, and Sokollu Mehmed began the preparations for the army's advance. The Grand Vizier went ahead, preparing for the arrival of the Sultan, who was leading the main part of the Ottoman forces. After 50 days, they arrived in Belgrade.