Eastern Hungarian Kingdom The Eastern Hungarian Kingdom ( in Hungarian : " Keleti Magyar Királyság " ) is the modern name used to designate the realm of John Zápolya and his son John Sigismund Zápolya , counter - kings to the Habsburg Hungarian Kings . There were several attempts to reunite the two Hungarian kingdoms under Habsburg rule , but the Turks prevented this by taking the Eastern Hungarian Kingdom under their protection . The exact territory of the polity was disputed because both kings ( the Habsburgs and the Zápolyas ) claimed the whole kingdom . Temporary territorial division occurred only at the secret negotiations of Treaty of Nagyvárad in 1538 . The Eastern Hungarian Kingdom is the predecessor of Principality of Transylvania that was established in 1570 in accordance with Treaty of Speyer ( ratified in 1571 ) . John I ' s reign . In 1526 , the main Hungarian army was defeated by the Ottomans at Battle of Mohács , Louis the Hungarian king was killed in action , however the Turks withdrew their troops and did not invade the country yet . The country became divided due to rival rulers . John I and Habsburg Ferdinand were supported by different factions of the nobility in the Hungarian kingdom . While John was the former voivode of Transylvania and the wealthiest and the most powerful landlord after Mohács , Ferdinand had the support of his brother the Emperor Charles . To counter this , John sought an alliance with the Ottomans and in 1528 the Ottoman sultan assented to an alliance with John I and gave written assurance of his support . He swore fealty to the sultan in 1529 . In 1538 , the two sides officialized the temporary division of the kingdom in accordance with Treaty of Nagyvárad , which was meant to last until the death of John Zápolya . The Habsburgs received a foothold in the north and west , Royal Hungary , with the new capital Pressburg . John II Sigismund ' s reign . In 1540 , when Zápolya died , his son , the infant John II Sigismund Zápolya was crowned by the Hungarian estates , and the kingdom remained divided . From 1541 or 1542 , the house of Zápolya also controlled the region that after 1571 became known as Partium . For much of John II ' s reign the country was governed by his mother , Isabella , with the help of the regent , the Catholic cleric George Martinuzzi and with the support of the sultan Suleiman the Magnificent . In 1541 the divided country fell apart into three parts because of the Turk occupation : a central portion controlled by the Ottoman Empire as Budin Province , a western part , called Royal Hungary , whose nobles elected Ferdinand as the king , in hope he would help expelling the Turks , and the Eastern Hungarian Kingdom , out of which later the Principality of Transylvania emerged . During the decade of 1540s , the Eastern Hungarian Kingdom included the counties of Máramaros , Szabolcs , Szatmár , Közép - Szolnok , Bihar , Külső - Szolnok , Békés , Csongrád , Arad , Csanád and the Temesköz . The bigger towns as Várad or Lippa were significant centres of state power , warranting predominance over the region ' s magnates . The one of the wealthiest noble Péter Petrovics was the absolute ruler of the Temesköz , however he was loyal to the Zápolya family . It followed that he cooperated with György Fráter ( George Martinuzzi ) the guardian of the infant John Sigismund . The regions from county of Máramaros to Kraszna river was ruled by the Drágffy - Perényi family , Ecsed and Somlyó by the Báthory family , Békés county by the Patócsy , the Maros river ' s valley by the Jaksics family and town of Debrecen by the Török family of Enying . The Zemplén , Borsod , and Abaúj counties with their undefined borders were ruled by the Balassa , Losonci , Bebek and the Drugeth s families however they possessed considerable autonomy . The army campaigns of 1543 – 44 left only one secure road link to Royal Hungary , along the Vág valley , and this momentum further decreased the numbers of Habsburg supporters in the kingdom . In August 1544 , commissioners from the central parts of the medieval Hungarian kingdom as counties along the Tisza river participated as equals in the Transylvanian diet at Torda . The Transylvanian diet became the legal successor of the Hungarian diets due to this practise . The chancellery and the high court at Buda disappeared during the political chaos of 1540 - 41 and Transylvania could not be administered by the central organs of the Hungarian Kingdom anymore . The apparatus of Transylvania ' s voivode was inadequate to provide the task of administering a state . György Fráter formed new administrative structure and he established the court at Gyulafehérvár . The feudal estates lost their leverage over the cases of state . The Saxons were still pro - Habsburg supporters and adopted a passive stance . Péter Haller , the royal magistrate at Szeben , was the only Saxon at the court of Gyulafehérvár . The Székelys had only few advocates in the circles around the regent and the queen . King John ' s supporters usually had no roots within the new confines of the country , however their relatives were found amongst the senior officials and courtiers in large numbers . The ruling class still trusted to the reunification of the country , and György Fráter always encountered the pressure of this wish and expectation . Habsburg rule and war . In 1549 , according to treaty of Nyírbátor , the legates of King Ferdinand I and of Isabella Jagiellon agreed to return Transylvania to the Habsburg Kingdom of Hungary . The treaty was Martinuzzi ' s work . Isabella , John Sigismund ' s mother , did not want to accept the dispossession of her son ' s crown and informed the Sultan immediately . Armed confrontations occurred between Isabella ' s forces and Martinuzzi ' s pro - Habsburg troops . Martinuzzi ' s army besieged the royal residence Gyulafehérvár two times in 1550 and 1551 . The agreement was signed by Isabella in 1551 . After the murder of György Fráter ( Martinuzzi ) by the Habsburgs in 1551 , Giovanni Battista Castaldo reoccupied Transylvania and the Tisza region for Habsburg Hungary . John Sigismund abdicated as King and , together with Isabella left for Poland . The sultan , feeling betrayed , ordered his army against Hungary in 1552 . Veszprém , Drégely , Szolnok , Lippa , Temesvár , Karánsebes and Lugos fell in the course of the campaign . Only Eger castle led by István Dobó could stand up to the Ottoman army . In 1553 , Ferdinand withdrew Castaldo ' s soldiers from Transylvania , however in 1554 the Sultan launched another attack against Hungary , occupying Salgó and Fülek . The recreated Zápolya ' s realm . In 1556 the nobles called King John II back and they elected him as Prince of Transylvania by the Diet of Szaszsebes , and Zápolya ' s realm was recreated . In 1568 , the freedom of religion was formally recognized and guarantied by John II Sigismund in the Edict of Torda . This religious tolerance and diversity had a lasting impact . In 1570 , John II Sigismund Zápolya , son of John I Zápolya renounced his claim as King of Hungary ( 1540 – 1570 ) in favour of Maximilian II of Habsburg , who also claimed the title since 1563 . Instead John II Sigismund Zápolya became the first Prince of Transylvania ( as " princeps Transsylvaniae et partium regni Hungariae dominus " - Prince of Transylvania and ruler of a part of the Kingdom of Hungary ) from 1570 until his death ( 1571 ) . Treaty of Speyer . In 1570 ( ratified in 1571 ) , by the Treaty of Speyer ( Spires ) , John II Sigismund , John I ' s son , abdicated as king of Hungary , and a new dukedom was invented for him : " " Joannes , serenissimi olim Joannis regis Hungariae , Dalmatiae , Croatiae etc . filius , Dei gratia princeps Transsylvaniae ac partium regni Hungariae " " ( imperial prince ) , from which derives the name Partium . This treaty , like the earlier Grosswardein ( Nagyvárad ) accord , endorsed the principle of a united Hungary . Partium and Transylvania were entrusted to John II Sigismund , but under the title of " imperial prince " . As mentioned above , the Zápolya held Partium before , but the treaty allowed them to do this without fear that the Habsburgs would contest the house of Zápolya ' s lordship . In a sense , Zápolya traded title for territory . Eastern Hungarian Kingdom became the predecessor to the Principality of Transylvania . The principality , where native princes , who paid the Turks tribute , ruled with considerable autonomy and where Austrian and Turkish influences vied for supremacy for nearly two centuries . All rulings after 1570 as King of Hungary refer to the territory known as " Royal Hungary " , and as Prince refer to the " Principality of Transylvania " .