of the Count of the Székelys in 1505 established a supreme court for Székely Land. Disintegration of Hungary Transylvania was mentioned as a regnum (or realm) in royal charters already in the second half of the 13th century. When writing of the regnum Transylvanum, the royal charters initially referred to the Transylvanian noblemen who formed a closed community "bound together by certain reciprocal rights and duties". The Székely and Saxon communities were included in the concept only after the confirmation of the Union of the Three Nations in 1459. The Tripartitumthe compendium of the customary law of Hungary, completed in 1514explicitly acknowledged that Transylvania was a separate realm, with her own peculiar customs, but it also emphasized that Transylvania was an integral part of the Kingdom of Hungary. The laws of the Kingdom of Hungary were to be applied in Transylvania and the decisions of the royal courts of justice were also to be obeyed in the province. The Ottoman Sultan, Suleiman the Magnificent, annihilated the royal army in the Battle of Mohács on 29 August 1529. Louis II of Hungary died and two candidates laid claim to the vacant throne. The majority of the noblemen elected the voivode of Transylvania, John Zápolya, king, but the wealthiest magnates offered the throne to Louis II's brother-in-law, Ferdinand of Habsburg, Archduke of Austria. During the ensuing civil war, the medieval Kingdom of Hungary was actually divided into two parts, with John Zápolya controlling the eastern regions, including Transylvania. Taking advantage of the turmoil which followed Zápolya's death in 1540, Suleiman conquered the central regions of the kingdom in late summer 1541. However, he allowed Zápolya's widow, Isabella Jagiellon, to continue to rule the lands to the east of the Tisza on behalf of her infant son, John Sigismund Zápolya, who had already been elected king at the initiative of his father's staunch supporter, George Martinuzzi. Martinuzzi held an assembly for the representatives of the privileged groups of John Sigismund's realm in Debrecen on 18 October 1541. This was the first Diet where both the Three Nations of Transylvania and the counties of Partium (the region between the Tisza and Transylvania) were represented. The delegates swore fealty to the Zápolyas and acknowledged the sultan's suzerainty. The following similar Diet was held only in 1544, but thereafter the delegates from the Partium were always invited to attend the Diet. Consequently, the Diet of John