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 Suleiman's side Ottoman attack. In early May, Sigismund went to Serbia, who was joined by Ban Jovan Morović from Mačva, but in June began Suleiman's new offensive. After fierce battles that were fought during the summer, Stefan withdrew and enclosed himself in Belgrade. He refused to conform with Suleiman, 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 Suleiman suzerainty, as did the Branković family. In addition to the conflict in Serbia, the year 1409 had several significant events that influenced the change of situation in the Balkans. Suleiman made peace with the Venetians in June, to whom they pledged to pay an annual tribute, as well as surrendering their former possessions in the area of Skadar and Zeta. His brother and rival in the struggle for power, Musa Çelebi moved to Europe and began to gather around him supporters and allies in the fight against Suleiman. Stefan and Musa Çelebi, as a result of conflict with Suleiman, were natural allies. Through his member, Duke Vitko, Stefan first checked Musa's force and then began negotiations that led to the conclusion of the alliance. In addition to Stefan,the Branković family, and Vuk Lazarević also had joined Musa. His forces began an offensive while Suleiman was in Asia Minor. At the beginning of 1410, Gallipoli was occupied, and by 13 February, were at Suleiman's Yambol, defeating defending beglerbeg Sinan, which has forced his brother to try to return to Europe and deal with Musa. That gave him the support of the Byzantine emperor Manuel II Palaiologos, with whom he remained on friendly terms and who gave him a boat to cross the Bosporus. Manuel's position significantly influenced the distribution of forces on the ground and very quickly changed those who supported Musa approach to Suleiman. Help in the transfer of his forces in Europe, tried to give prince Vuk, who asked the Venetian deputies in early June to obtain the Venetian fleet's assistance in the transport of troops.