In 1390, Stefan Lazarević of Serbia accepted the Ottoman sultan's suzerainty, thus the Ottoman Empire's expansion reached the southern frontiers of Hungary.^[57] Sigismund decided to organize a crusade against the Ottomans.^[58] A great army consisting mainly of French knights assembled, but the crusaders were routed in the battle of Nicopolis in 1396.^[59]

The Ottomans occupied Golubac Fortress in 1427 and started to regularly plunder the neighboring lands.^[60] The northern regions of the kingdom (present-day Slovakia) were pillaged in almost every year by Czech Hussites from 1428.^[61] However, Hussite ideas spread in the southern counties, mainly among the burghers of the Szerémség. Hussite preachers were also the first to translate the Bible to Hungarian. However, all Hussites were either executed or expelled from the Szerémség in the late 1430s.^[62]