Eastern Hungarian Kingdom in the context of "Royal Hungary"

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 Battle of Mohács (Hungarian: [ˈmohaːt͡ʃ]; Hungarian: mohácsi csata, Turkish: Mohaç Muharebesi) took place on 29 August 1526 near Mohács, in the Kingdom of Hungary. It was fought between the forces of Hungary, led by King Louis II, and the invading Ottoman Empire, commanded by Suleiman the Magnificent and his grand vizier, Pargalı Ibrahim Pasha. The Ottomans achieved a decisive victory through superior planning, firepower, and a well-executed encirclement that overwhelmed the Hungarian forces.

The Hungarian army, encouraged by the nobility to engage prematurely, launched a frontal assault that collapsed under coordinated Ottoman counterattacks. King Louis and much of the Hungarian aristocracy were killed, resulting in the destruction of the royal army and the end of the Jagiellonian dynasty in Hungary and Bohemia. The aftermath saw the partition of Hungary between the Ottoman Empire, the Habsburg monarchy, and the Eastern Hungarian Kingdom in 1541.

Alba Iulia (Romanian pronunciation: [ˌalba ˈjuli.a] ; German: Karlsburg or Carlsburg, formerly Weißenburg [ˈvaɪsn̩bʊʁk] ; Hungarian: Gyulafehérvár [ˈɟulɒfɛɦeːrvaːr]; Latin: Apulum) is a city that serves as the seat of Alba County in the west-central part of Romania. Located on the river Mureș in the historical region of Transylvania, it has a population of 64,227 (as of 2021).

During ancient times, the site was the location of the Roman camp Apulum. Since the High Middle Ages, the city has been the seat of Transylvania's Roman Catholic diocese. Between 1526 and 1570 it was the capital of the Eastern Hungarian Kingdom from which the Principality of Transylvania emerged by the Treaty of Speyer in 1570 and it was the capital of the Principality of Transylvania until 1711. At one point it also was a center of the Eastern Orthodox Metropolitan of Transylvania with suffragan to Vad diocese. On 1 December 1918, the Union of Transylvania with Romania was declared in Alba Iulia, and Romania's King Ferdinand I and, in 1922 Queen Marie were crowned in the Alba Iulia Orthodox Cathedral.

The Habsburg monarchy and the Ottoman Empire waged a series of wars on the territory of the Kingdom of Hungary and several adjacent lands in Southeastern Europe from 1526 to 1568. The Habsburgs and the Ottomans engaged in a series of military campaigns against one another in Hungary between 1526 and 1568. While overall the Ottomans had the upper hand, the war failed to produce any decisive result. The Ottoman army remained very powerful in the open field but it often lost a significant amount of time besieging the many fortresses of the Hungarian frontier and its communication lines were now dangerously overstretched. At the end of the conflict, Hungary had been split into several different zones of control, between the Ottomans, Habsburgs, and Transylvania, nominally Ottoman vassal state. The simultaneous war of succession between Habsburg-controlled western "Royal Hungary" and the Zápolya-ruled pro-Ottoman "Eastern Hungarian Kingdom" is known as the Little War in Hungary.

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.