The son of Serbian Emperor, Manuel, who asisted Bayezid as an obedient vassal of the sultan during the Karaman campaign, went to Constantinople, without permission. On hearing this Bayezid thought that this journey should had have a secret aim. Therefore, he cancelled his Hungary campaign and decided to besiege Constantinople.

He besieged the city both from the land and the sea (1391). But the Ottomans could not have managed to breakthrough the massive walls of the city and the siege had turned out to be just a blockade. With the Hungarian raid to the Turkish lands, the siege was withdrew.

The Hungarians commanded by King Sigismund attacked the Ottoman territories from the north (1392). The Turkish raiders were sent against them. After the Danube-Wallachia campaign, Salonica and its province were annexed (1394). Bayezid besieged Constantinople for the second time in 1395, because he was informed that another Crusade was emerged against him, this siege was failed, too.

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