In 1389 was fought the great battle of Kosovo Polje, or the Field of Blackbirds, a large plain in Old Serbia, at the southern end of which is Skoplje. At this battle Serbian armies from all the Serb lands, including Bosnia, joined together in defence of their country for the last time. The issue of the battle was for some time in doubt, but was decided by the treachery and flight at the critical moment of one of the Serb leaders, Vuk Branković, son-in-law of Prince Lazar, with a large number of troops. Another dramatic incident was the murder of Sultan Murad in his tent by another Serbian leader, Miloš Obilić, who, accused of treachery by his own countrymen, vowed he would prove his good faith, went over to the Turks and, pretending to be a traitor, gained admission to the Sultan’s presence and proved his patriotism by killing him. The momentary dismay was put an end to by the energetic conduct of Bayezid, son of Murad, who rallied the Turkish troops and ultimately inflicted total defeat on the Serbians. From the effects of this battle Serbia never recovered; Prince Lazar was captured and executed; his wife, Princess Milica, had to give her daughter to Bayezid in marriage, whose son thus ultimately claimed possession of Serbia by right of inheritance. Princess Milica and her son Stephen continued to live at Kruševac, but Serbia was already a tributary of Turkey. In the north, Hungary profited by the course of events and occupied Belgrade and all northern Serbia, but in 1396 the Turks defeated the Magyars severely at the battle of Nikopolis, on the Danube, making the Serbs under Stephen fight on the Turkish side. Stephen also had to help Sultan Bajazet against the Tartars, and fought at the battle of Angora, in 1402, when Tamerlane captured Bayezid. After Stephen returned to Serbia he made an alliance with Hungary, which gave him back Belgrade and northern Serbia; it was at this time (1403) that Belgrade first became the capital, the political centre having in the course of fifty years moved from the Vardar to the Danube. The disorders which followed the defeat of Bayezid gave some respite to the Serbs, but Sultan Murad II (1421-51) again took up arms against him, and invaded Serbia as far as Kruševac.