Principality of Transylvania (1570–1711) replaces Eastern Hungarian Kingdom
Background
Eastern Hungarian Kingdom and Zápolya family
On 29 August 1526, the army of Sultan Suleiman of the Ottoman Empire inflicted a decisive defeat on the Hungarian forces at Mohács. John Zápolya was en route to the battlefield with his sizable army but did not participate in the battle for unknown reasons. The youthful King Louis II of Hungary and Bohemia fell in battle, as did many of his soldiers. When Zápolya was proclaimed king of Hungary, Ferdinand from the House of Habsburg also claimed the throne. In the ensuing struggle John Zápolya received the support of Sultan Suleiman I, who after Zápolya's death in 1540, occupied Buda and central Hungary in 1541 under the pretext of protecting Zápolya's son, John II. Hungary was now divided into three sections: Royal Hungary in the west and north, Ottoman Hungary, and the Eastern Hungarian Kingdom under Ottoman suzerainty, which later became the Principality of Transylvania, where Austrian and Turkish influences vied for supremacy for nearly two centuries. The Hungarian magnates of Transylvania resorted to a policy of duplicity in order to preserve independence.
Transylvania was administrated by Isabella, John Sigismund's mother, from 1541 to 1551, when it fell for five years under Habsburg rule (1551–1556). The House of Zapolya regained the control of Transylvania in 1556, when the Diet of Szászsebes elected Sigismund as prince of Transylvania.
Transylvania was now beyond the reach of Catholic religious authority, allowing Lutheran and Calvinist preaching to flourish. In 1563, Giorgio Blandrata was appointed as court physician, and his radical religious ideas increasingly influenced both the young king John II and the Calvinist bishop Francis David, eventually converting both to the Anti-Trinitarian (Unitarian) creed. In a formal public disputation, Francis David prevailed over the Calvinist Peter Melius; resulting in 1568 in the formal adoption of individual freedom of religious expression under the Edict of Torda. This was the first such legal guarantee of religious freedom in Christian Europe, but only for Lutherans, Calvinists, Unitarians and of course Catholics, the Orthodox Christian confession being "tolerated" with no legal guarantees granted.