Stefan and Musa Çelebi, as a result of conflict with Süleyman, 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 Süleyman was in Asia Minor. At the beginning of 1410, Gallipoli was occupied, and by 13 February, were at Süleyman's Yambol, defeating defending beglerbeg Sinan, which has forced his brother to try to return to Europe and deal with Musa.