The Eastern Hungarian Kingdom () is a modern term coined by some historians to designate the realm of John Zápolya and his son John Sigismund Zápolya, who contested the claims of the House of Habsburg to rule the Kingdom of Hungary from 1526 to 1570. The Zápolyas ruled over an eastern part of Hungary, and the Habsburg kings (Ferdinand and Maximilian) ruled the west. The Habsburgs tried several times to unite all Hungary under their rule, but the Ottoman Empire prevented that by supporting the Eastern Hungarian Kingdom.
The exact extent of the Zápolya realm was never settled because both the Habsburgs and the Zápolyas claimed the whole kingdom. A temporary territorial division was made in the Treaty of Nagyvárad in 1538. The Eastern Hungarian Kingdom is considered by some historians the predecessor of the Principality of Transylvania (1570–1711), which was established by the Treaty of Speyer.
John I's reign
In 1526, Hungary was defeated by the Ottoman Empire at the Battle of Mohács, and King Louis II of Hungary was killed. The Ottomans withdrew their troops and did not then seek to conquer.
Ferdinand of Austria, younger brother of Emperor Charles V, claimed the crown by right of his marriage to Louis' sister Anne. But most Hungarian nobles opposed Ferdinand. They supported John Zápolya, former Voivode of Transylvania, the wealthiest landholder in the country. The Hungarian Diet proclaimed him king, but Ferdinand sent an army which drove John from the country by 1528. To counter the Habsburg influence, John formed an alliance with Ottoman Sultan Suleiman I in 1528, and even swore fealty to the sultan in 1529.
John controlled Transylvania and the eastern part of the Hungarian plain; Ferdinand held Croatia, the western part of the plain, and Upper Hungary.
In 1538, the two sides signed the Treaty of Nagyvárad, which made this division official, and also made Ferdinand heir to John, who was childless.
John II Sigismund's reign
The Nagyvárad agreement lasted only two years, until John I's son was born in 1540, only nine days before John I's death. The infant John II Sigismund Zápolya was crowned by the Hungarian estates a few weeks later. For much of John II's reign, Eastern Hungary was governed by his mother, Isabella, with Bishop George Martinuzzi as regent. They were supported by Sultan Suleiman, who recognized John II as king and his vassal.
In 1541, Ferdinand invaded to enforce his claim. Martinuzzi called on Suleiman, who expelled Ferdinand, but took most of central Hungary under direct Ottoman rule as Budin Province. The eastern part of the Hungarian plain remained under Zápolya rule; after 1571 it became known as Partium.
During the 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, such as Várad or Lippa, were significant centers of state power, warranting predominance over the region's magnates. One of the wealthiest nobles, Péter Petrovics, was the absolute ruler of the Temesköz, but was loyal to the Zápolya family. He cooperated with the regent Martinuzzi. The region from Máramaros County to the Kraszna river was ruled by the Drágffy-Perényi family, Ecsed and Somlyó by the Báthory family, Békés county by the Patócsy, the Maros river valley by the Jaksics family, and the town of Debrecen by the Török family of Enying. Zemplén, Borsod, and Abaúj Counties with their undefined borders were ruled by the Balassa, Losonci, Bebek, and Drugeth families, but 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 further decreased Habsburg support 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 thus became the legal successor of the Hungarian Diets.
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 the voivode was inadequate to provide the task of administering a state. Martinuzzi formed a new administrative structure, and established the court at Gyulafehérvár.