Stefan had a quarrel with his brother Vuk Lazarević, who went to the Ottoman side, to the new sultan (actually co-ruler with his three brothers) Suleyman (I) Çelebi. Counting on unrest within the Ottoman Empire (Ottoman Interregnum), in early 1404 Stefan accepted vassalage to the Hungarian king Sigismund, who awarded him with Belgrade, the Mačva region, and the fort of Golubac, until then in possession of the Kingdom of Hungary, so Belgrade became a capital of Serbia for the second time in history after King Dragutin. He managed to liberate his sister and Bayezid's widow Olivera. In 1404 he made peace with his brother Vuk, in 1405 he married Caterina Gattilusio, daughter of Francesco II Gattilusio, ruler of the island of Lesbos. Also in 1405 his mother Milica died.