According to the Treaty of Nagyvárad, John Zápolya was to be succeeded by Archduke Ferdinand of Austria, but Hungarian nobles, particularly Bishop George Martinuzzi, refused to abide by it and in September 1540 elected the infant John Sigismund as King of Hungary and Isabella as his regent. Ferdinand invaded Hungary and besieged Buda where Isabella and her son sought shelter. The city withstood the first siege in November 1540 and the second siege in May 1541 was lifted by Suleiman the Magnificent, Sultan of the Ottoman Empire. She did not receive help from her father Sigismund I the Old. Suleiman preferred a fragmented Hungary but understood that only he could protect it from the Habsburgs. Therefore, he converted most of Hungary to pashaliks (province governed by a pasha) and only Transylvania and lands east of the Tisza River (known as Temesköz) were given to John Sigismund and Isabella as his guardian. These appointments were accepted by the Transylvanian Diet acknowledging its status as a vassal of the Ottoman Empire.