In revenge for the death by stealth of Murad in the Battle of Kosovo, Beyazid massacred his Serb prisoners. Nevertheless, he was able to conclude a treaty with the Serb leader, Stephen Bulcovic, and granted Serbia considerable autonomy. In 1391 he laid siege to Constantinople. On the demand of the Byzantine emperor John V Palaeologus a new Crusade was organized to defeat Beyazid. In 1396, the Christian allies, under the leadership of the Hungarian King and Holy Roman Emperor Sigismund, was defeated in the Battle of Nicopolis. The siege of Constantinople lasted until 1398 when the Byzantines paid a considerable tribute to break the siege.
In 1400, the Mongol warlord Timur Lenk had succeeded in rousing the local kingdoms that had been conquered by the Turks to join him in his attack on Beyazid. In the fateful Battle of Ankara, on July 20, 1402, Beyazid was captured by Timur and kept chained in a cage as a trophy. There are many stories about Beyazid's captivity, including one that describes how Timur used him as a footstool. One year later, Beyazid died -- some accounts claim that he committed suicide.
1389 to 1402