János Hunyadi, a Magyarized Romanian from Transylvania, had become acquainted with the most up-to-date military strategy and implements of war while serving in Italian and Czech mercenary armies as well as that of Emperor Sigismund.

When Hungary falls into a period of turmoil after Sigismund's death in 1437, the Ottoman Turks, advancing into Europe and Hungarian territory, move along the Danube River to the south of heavily-fortified Belgrade and besiege the Semendria (Smederjevo) fortress in 1437.

Hunyadi, now thirty, leads Hungarian forces in a spectacularly successful relief operation, evicting the Turks.