The Ottoman Sultan Suleiman the Magnificent () nearly annihilated the Hungarian army in the Battle of Mohács on 29 August 1526. The twenty-year-old King Louis II () drowned in a stream fleeing from the battlefield. John Zápolya and Louis's brother-in-law, Ferdinand of Habsburg, laid claim to the throne. For both claimants were elected kings at two separate Diets, Hungary was plunged into civil war. Regarding Hungary as a vassal state, Suleiman acknowledged John as the lawful king, but neither claimants could take control of the whole country. The two kings agreed to divide the country in the 1538 Treaty of Várad (Oradea, Romania), but the childless John acknowledged Ferdinand's right to reunite Hungary after his death. Days before John died in 1540, his wife, Isabella Jagiellon, gave birth to a son, John Sigismund. John's principal advisor, the Paulician monk, George Martinuzzi, persuaded John's partisans to elect the infant boy king. Ferdinand's troops stormed into John Sigismund's eastern Hungarian kingdom, providing Suleiman with a pretext to intervene. The Ottoman army invaded Hungary and seized the kingdom's capital, Buda, without resistance on 29 August 1541. Suleiman confirmed John Sigismund's right to rule the land to the east of the river Tisza and the Ottoman troops conquered the kingdom's central regions.