Buda, the capital of Hungary, fell to the Ottomans without opposition in 1541, but Suleiman allowed the dowager queen, Isabella, to retain the territory east of the river Tisza on John Sigismund's behalf.

John Sigismund's realm was administered by his father's treasurer, George Martinuzzi, who sought to reunite Hungary under the rule of Ferdinand.

At Suleiman's urging, the Transylvanian Diet in 1556 persuaded John Sigismund and his mother to return to Transylvania, where she ruled her son's realm until her death in 1559.

The 1568 Treaty of Adrianople concluded the war, confirming John Sigismund in the eastern territories of the medieval Kingdom of Hungary (Transylvania and "Partium").

Around five years later, accepting the Anti-Trinitarian views of his physician, Giorgio Biandrata and court preacher Ferenc Dávid, he became the only Unitarian monarch in history.

In 1568, the Diet passed the Edict of Torda (now Turda in Romania), which emphasized that "faith is a gift of God" and prohibited the persecution of people for religious reasons.

[1] After the Ottoman Sultan, Suleiman the Magnificent, inflicted a crushing defeat on the Hungarian army in the Battle of Mohács, the majority of the noblemen elected John Zápolya king in 1526.

[7][13] The Diet proclaimed Queen Isabella and George Martinuzzi, along with two powerful lords, Péter Petrovics and Bálint Török, the guardians of the infant monarch.

[15][16] On his command, Petru Rareș, Prince of Moldavia, captured Stephen Majláth and forced the Transylvanian Diet to swear fealty to John Sigismund in late July.

[17] Suleiman said that he had come to protect John Sigismund's interests, but also announced that he wanted to see the infant king, because he had heard rumours about Isabella's having actually given birth to a daughter.

[26] In the same month, Saxon clergymen from Kronstadt (now Brașov in Romania), who had adopted Lutheranism, participated in a debate with Catholic priests in the presence of the queen and Martinuzzi in Gyulafehérvár.

[29] In April 1544, the Diet of Torda prescribed that travellers should respect the religious customs of the settlements they visited, showing that the ideas of the Reformation had spread to the whole province.

[31] Five counties that had previously accepted Ferdinand's rule – Bereg, Szabolcs, Szatmár, Ung and Zemplén – swore allegiance to John Sigismund before the end of 1555.

[27] The sultan refused to include John Sigismund's realm in the peace treaty that he concluded with Ferdinand's brother, Emperor Charles V, in 1547.

[32] Both actions suggested that Suleiman intended to seize part of John Sigismund's kingdom, prompting Isabella and Martinuzzi to reopen negotiations with Ferdinand on the reunification of Hungary in 1548.

[33][34] According to their agreement, Isabella and John Sigismund were to abdicate in exchange for the Silesian duchies of Opole and Racibórz and 100,000 florins in compensation.

[36] Isabella and her supporters Péter Petrovics and Ferenc Patócsy made a new attempt to prevent the execution of the Treaty of Nyírbátor in May 1551, but Martinuzzi defeated them.

[42] The contemporaneous historian Ferenc Forgách, who was Isabella's implacable enemy, accused her of bringing her son up "shamefully", allowing him to keep bad company and drink.

[46] Deciding in April 1554 that Hungary should be restored to John Sigismund,[47] Suleiman allowed Péter Petrovics to take control of two fortresses in Banat.

[47] Henry II of France, engaged in war against the Habsburgs, also urged Isabella to return to Hungary, promising one of his daughters in marriage to John Sigismund.

[55] A year and a half later, John Sigismund sent letters to the University of Wittenberg and other theological centers in Germany to seek advice on the main points of the two Protestant schools of thought.

[63] Ferenc Dávid began to include Anti-Trinitarian ideas in his sermons, which infuriated the Calvinist bishop of Debrecen, Péter Melius Juhász.

[67] In a letter written about this time to Cosimo I, Duke of Florence, the mercenary Giovanandrea Gromo described John Sigismund as "extremely benevolent, gracious, subtle in thought, wise, level-headed, industrious [and] brave".

[41][54] Gromo mentioned that John Sigismund spoke Latin, Italian, German, Polish, Hungarian and Romanian well, and could also speak Greek and Turkish.

... [H]is blue eyes gaze mildly and with benevolence ... His arms and hands are long and finely articulated, but powerful ... [H]e heartily enjoys every kind of hunting, both for large game ... and for hare and fowl.

Among his recognized fine qualities is his abstinent mode of life ...John Sigismund appointed a Calvinist bishop as the only religious leader of the Romanians in his realm in November 1566.

[77] The decree did not put a complete end to discrimination, because official status was granted only to the Catholic, Lutheran, and Calvinist clergymen, but Unitarian, Orthodox, Armenian, Jewish, and Muslim believers could also freely practise their religions.

[62] According to historian Gábor Barta, political factors also contributed to John Sigismund's conversion, because he "found in [Anti-Trinitarianism] the means through which he could express both his adherence to the Christian world and the distance from it".

[41] John Sigismund had made his last testament and will in the presence of Chancellor Mihály Csáky and Treasurer Gáspár Bekes during his earlier illness in the summer of 1567.

[92] Gáspár Bekes, supported by Maximilian II, contested the election, but Báthory emerged victorious in the resulting civil war and consolidated his rule.