John Zápolya in the context of "Voivode of Transylvania"

The Kingdom of Hungary between 1526 and 1867 existed as a state outside the Holy Roman Empire, but part of the lands of the Habsburg monarchy that became the Austrian Empire in 1804. After the Battle of Mohács in 1526, the country was ruled by two crowned kings (John I and Ferdinand I). Initially, the exact territory under Habsburg rule was disputed because both rulers claimed the whole kingdom. This unsettled period lasted until 1570 when John Sigismund Zápolya (John II) abdicated as King of Hungary in Emperor Maximilian II's favor.

In the early stages, the lands that were ruled by the Habsburg Hungarian kings were regarded as both the "Kingdom of Hungary" and "Royal Hungary". Royal Hungary was the symbol of the continuity of formal law after the Ottoman occupation, because it could preserve its legal traditions, but in general, it was de facto a Habsburg province. The Hungarian nobility forced Vienna to admit that Hungary was a special unit of the Habsburg lands and had to be ruled in conformity with its own special laws. However, Hungarian historiography positioned Transylvania in a direct continuity with the medieval Kingdom of Hungary in pursuance of the advancement of Hungarian interests.

The siege of Vienna, in 1529, was the first attempt by the Ottoman Empire to capture the city of Vienna in the Archduchy of Austria, part of the Holy Roman Empire. Suleiman the Magnificent, sultan of the Ottomans, attacked the city with over 100,000 men, while the defenders, led by Niklas Graf Salm, numbered no more than 21,000. Nevertheless, Vienna was able to survive the siege, which ultimately lasted just over two weeks, from 27 September to 15 October 1529.

The siege came in the aftermath of the 1526 Battle of Mohács, which had resulted in the death of Louis II, King of Hungary, and the descent of the kingdom into civil war. Following Louis' death, rival factions within Hungary selected two successors: Archduke Ferdinand I of Austria, supported by the House of Habsburg, and John Zápolya. Zápolya would eventually seek aid from, and become a vassal of the Ottoman Empire, after Ferdinand began to take control of western Hungary, including the city of Buda.

The Hundred Years' Croatian–Ottoman War (Croatian: Stogodišnji hrvatsko-turski rat, Stogodišnji rat protiv Turaka, Stogodišnji rat s Osmanlijama) was a sequence of conflicts, mostly of relatively low intensity ("Small War", Croatian: Mali rat), between the Ottoman Empire and the medieval Kingdom of Croatia (ruled by the Jagiellon and Zápolya dynasties), and the later Habsburg Kingdom of Croatia. Besides periods of small-scale borderland warfare, the conflict also saw episodes of major conquest campaigns of Croatian land which was undertaken by the Ottomans especially during the 16th century.

The expansion of the Ottoman Empire in Europe was stopped in the Battle of Sisak 1593. Nevertheless, the Ottoman Empire held control over parts of Croatia from the 16th to the end of the 17th century, when most of the territories were regained in the Great Turkish War, except for lands known as Turkish Croatia (roughly corresponding to modern day western Bosnia and Herzegovina) which remained in Ottoman hands until the 19th century.

John Sigismund Zápolya or Szapolyai (Hungarian: Szapolyai János Zsigmond; 7 July 1540 – 14 March 1571) was King of Hungary as John II from 1540 to 1551 and from 1556 to 1570, and the first Prince of Transylvania, from 1570 to his death. He was the only son of John I, King of Hungary, and Isabella of Poland. John I ruled parts of the Kingdom of Hungary with the support of the Ottoman Sultan Suleiman; the remaining areas were ruled by Ferdinand I of Habsburg, who also ruled Austria and Bohemia. The two kings concluded a peace treaty in 1538 acknowledging Ferdinand's right to reunite Hungary after John I's death, though shortly after John Sigismund's birth, and on his deathbed, John I bequeathed his realm to his son. The late king's staunchest supporters elected the infant John Sigismund king, but he was not crowned with the Holy Crown of Hungary.

Suleiman invaded Hungary under the pretext of protecting John Sigismund from Ferdinand. 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. Isabella and John Sigismund moved to Lippa (now Lipova in Romania). Before long, they took up residence in Gyulafehérvár in Transylvania (Alba Iulia in Romania). John Sigismund's realm was administered by his father's treasurer, George Martinuzzi, who sought to reunite Hungary under the rule of Ferdinand. Martinuzzi forced Isabella to renounce her son's realm in exchange for two Silesian duchies and 140,000 florins in 1551. John Sigismund and his mother settled in Poland, but she continued to negotiate for John Sigismund's restoration with Ferdinand's enemies.

The Eastern Hungarian Kingdom (Hungarian: keleti Magyar Királyság [ˈkɛlɛti ˈmɒɟɒr ˈkiraːjʃaːg]) 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 the predecessor of the Principality of Transylvania (1570–1711), which was established by the Treaty of Speyer.

Isabella Jagiellon (Hungarian: Izabella királyné; Polish: Izabela Jagiellonka; Lithuanian: Izabelė Jogailaitė; 18 January 1519 – 15 September 1559) was a princess of the Kingdom of Poland and of the Grand Duchy of Lithuania, and later also the Queen consort of Hungary. She was the oldest child of Sigismund I the Old, King of Poland and Grand Duke of Lithuania, and his Italian wife Bona Sforza.

In 1539, she married John Zápolya, Voivode of Transylvania and King of Hungary. At the time Hungary was contested between Ferdinand of Austria who wanted to add it to the Habsburg domains (see Royal Hungary), local nobles who wanted to keep Hungary independent (see Eastern Hungarian Kingdom), and Sultan Suleiman the Magnificent who saw it as a vassal state of the Ottoman Empire (see also Little War in Hungary). While Isabella's marriage lasted only a year and a half, it did produce a male heir – John Sigismund Zápolya born just two weeks before his father's death in July 1540.

George Martinuzzi, O.S.P.P.E. (born Juraj Utješenović, also known as György Martinuzzi, Brother György, Georg Utiessenovicz-Martinuzzi or György Fráter, Hungarian: Fráter György; 1482 – 16 December 1551), was a Croatian nobleman, Pauline monk and Hungarian statesman who supported King John Zápolya and his son, King John Sigismund Zápolya. He was Bishop of Nagyvárad (now Oradea), Archbishop of Esztergom and a cardinal.