After the death of John Zápolya, king of Hungary as an Ottoman vassal, in 1540, Ferdinand I, Holy Roman Emperor looked to annex Zápolya's lands (the Eastern Hungarian Kingdom). The Hungarian diet had elected infant John Sigismund Zápolya, the son of Zápolya and Isabella Jagiellon, as King of Hungary, which broke the Treaty of Nagyvárad, and Ferdinand I invaded Hungary. Queen Isabella struggled to rule Hungary for her son. Frater George Martinuzzi, appointed by John as regent, opposed her (he would later be created a Cardinal as reward for his accomplishments in this conflict). Ferdinand I sent mercenary leader ("condottiero") Bartolomeo Castoldo with more than 7,000 mercenaries who beat a contingent led by Péter Petrovics, killing more than 2,500 of them, near Csanád. The Sultan immediately ordered Sokollu Mehmed Pasha to move into Hungary, so he assembled an army of 90,000 soldiers and fifty-four cannons and also summoned the pashas of Smederevo, Vidin and Nicopolis. When his forces reached Slankamen in Syrmia, George Martinuzzi begged Mehmed not to attack Transylvania, arguing that it had remained in the possession of the Sultan. Mehmed rejected negotiation proposals, led Ottoman forces into Transylvania and soon captured 16 cities, including Bečej, Becskerek, Csanád and Lippa. In this campaign, Sokollu won over to his side local Serb-manned garrisons by pointing out to his common ethnicity with them. Martinuzzi responded by raising a rebellion in Transylvania, mustering one soldier from every household. Mehmed had to fall back and once again laid siege to Temesvár on 14 October with the main part of his army and 50 cannons. Mehmed demanded surrender, but the city's commander, István Losonci, replied with a recommendation for Mehmed's return to Rumelia.