 Complete the following phrase:  According to the Treaty of Nagyvárad, John Zápolya was to be succeeded by Archduke Ferdid 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. Ferdid invaded Hungary and besieged Buda where Isabella and her son sought shelter. The city withstood the first siege in November 1540 and the 