At the beginning of 1409, Süleyman's Ottoman forces broke into Serbia at the battlefield of Kosovo and nearby Priština was destroyed, as evidenced by a letter that arrived in February in Dubrovnik, from the merchants of the city. Dubrovnik people in Serbia were also instructed that, as citizens of the Republic, could call for its neutrality during the conflict, but they were also told not to harm Stefan's people, as well as in the case of attacks on towns where they were engaged in their defense. Stefan was assisted in the fighting by Sigismund I, whose forces were under the command of Philip de Skolarisa, late January through Kovin, joined to Serbia. His quick reaction testifies to the fact that Stefan and Sigismund were aware of Vuk's impending departure to Süleyman's side Ottoman attack. In early May, Sigismund went to Serbia, who was joined by Ban Jovan Morović from Mačva, but in June began Süleyman's new offensive. After fierce battles that were fought during the summer, Stefan withdrew and enclosed himself in Belgrade. He refused to conform with Süleyman, but was forced to negotiate with his brother, which practically led to the division of the country. Vuk was submitted to the administration of its southern part, which included the area south of the West Morava. He ruled on his own and accepted Süleyman suzerainty, as did the Branković family.