Szapolyai didn't have a son and according to treaty of Nagyvárad signed in 1538, Ferdinand was the heir to throne. But after the treaty, Szapolyai's wife gave birth to a son. In 1540, when Szapolyai died of natural causes, Ferdinand reclaimed the throne, and the war was renewed.[5] This time, Suleyman reversed his policy of allowing Hungary to persist as a vassal kingdom, and annexed most of Hungary in his two campaigns in 1541 and 1543.