Hungary and the Ottomans
According to the terms set at the First Congress of Vienna in 1515, Ferdinand married Anne Jagiellonica, daughter of King Vladislaus II of Hungary and Bohemia, on 22 July. The Kingdom of Hungary and Kingdom of Bohemia were both elective monarchies, where the parliaments had the sovereign right to choose the king. When his brother-in-law Louis II of Hungary and Bohemia was killed at the Battle of Mohács against the Ottoman Empire on 29 August 1526, Ferdinand immediately declared himself a candidate in the royal elections. On 24 October 1526, the Bohemian Diet, acting under the influence of chancellor Adam of Hradce, elected Ferdinand king of Bohemia under conditions of confirming traditional privileges of the estates and also moving the Habsburg court to Prague. The success was only partial, as the Diet refused to recognise Ferdinand as hereditary ruler of the kingdom.
The throne of Hungary was disputed between Ferdinand and John Zápolya, Voivode of Transylvania. They were supported by different factions of the Hungarian nobility. Ferdinand also had the support of his brother, Emperor Charles V. Nicolaus Olahus, secretary of Louis, attached himself to Ferdinand's part, but retained his position with his sister, Queen Dowager Mary of Hungary.
On 10 November 1526, in Székesfehérvár, John Zápolya was proclaimed king by a Hungarian Diet formed of the untitled lesser nobility (gentry).
On 17 December 1526, in Pozsony (Bratislava in Slovak), Ferdinand was elected King of Hungary, Dalmatia, Croatia, Slavonia, etc. by a rump Diet the higher aristocracy (the magnates or barons) and the Hungarian Catholic clergy. Accordingly, Ferdinand was crowned as King of Hungary in the Székesfehérvár Basilica on 3 November 1527.
The Croatian nobles unanimously accepted the Pozsony election of Ferdinand I, receiving him as their king in the 1527 election in Cetin, and confirming the succession to him and his heirs. In return for the throne, King Ferdinand promised to respect the historic rights, freedoms, laws, and customs of the Croats when they united with the Hungarian kingdom and to defend Croatia from Ottoman invasion.
Brendan Simms notes that Ferdinand got the Hungarian and Bohemian crowns instead of his brother, Emperor Charles V, because the Emperor was busy fighting the Ottomans in Hungary, Austria, and the Mediterranean Sea, while also fighting against France and German Protestants in Italy and Germany. Thus, the defense of central Europe was delegated to Ferdinand; he also had many responsibilities in the management of the empire. Charles V abdicated as archduke of Austria in 1522, and nine years after that, he had the German princes elect Ferdinand as King of the Romans, who thus became his designated successor.
"This had profound implications for state formation in south-eastern Europe. Ferdinand rescued Bohemia and Silesia from the Hungarian wreckage, making his north-eastern flank more secure. He told the Austrian Landtag, the assembled representatives of the nobility, at Linz in 1530, that "the Turks cannot be resisted unless the Kingdom of Hungary is in the hands of an Archduke of Austria or another German prince". After some hesitation, Croatia and the Hungarian rump joined the Habsburgs. In both cases, the link was essentially a contractual one, directly linked to Ferdinand's ability to provide protection against the Turks".
The Austrian lands were in miserable economic and financial conditions, but Ferdinand was forced to introduce the so-called Turk Tax (Türkensteuer) for the war against the Ottomans. In spite of huge Austrian sacrifices, he could not raise enough money. His annual revenues only allowed him to hire 5,000 mercenaries for two months. So Ferdinand asked for help from his brother the Emperor Charles V and borrowed from bankers like the Fugger family.
Ferdinand defeated Zápolya at the Battle of Tarcal in September 1527 and again in the Battle of Szina in March 1528. John Zápolya fled the country and applied to Sultan Suleiman the Magnificent for support.
This led to the most dangerous moment of Ferdinand's career, in 1529, when Suleiman took advantage of this Hungarian support for a massive but ultimately unsuccessful assault on Ferdinand's capital: the Siege of Vienna, which sent Ferdinand into refuge in Bohemia. A further Ottoman invasion was repelled in 1532 (see Siege of Güns). Ferdinand then made peace with the Ottomans. The Treaty of Constantinople (1533) divided Hungary between the Habsburg kingdom in the west and John Zápolya's Eastern Hungarian Kingdom, a vassal state of the Ottoman Empire.
Together with the formation of the Schmalkaldic League in 1531, this struggle with the Ottomans caused Ferdinand to grant the Nuremberg Religious Peace. As long as he hoped for a favorable response from his humiliating overtures to Suleiman, Ferdinand was not inclined to grant the peace which the Protestants demanded at the Diet of Regensburg, which met in April 1532. However, as the army of Suleiman drew nearer, he yielded. On 23 July 1532, the peace was concluded at Nuremberg, where the final deliberations took place. Those who had up to this time joined the Reformation obtained religious liberty until the meeting of a council, and in a separate compact, all proceedings in matters of religion pending before the imperial chamber court were temporarily paused.
In 1538, in the Treaty of Nagyvárad, Ferdinand induced the childless Zápolya to name him as his successor. But in 1540, just before his death, Zápolya had a son, John Sigismund Zápolya, who was promptly elected king by the Diet. Ferdinand invaded Hungary, but the regent, Frater George Martinuzzi, Bishop of Várad, called on the Ottomans for protection. Suleiman marched into Hungary (see Siege of Buda (1541)) and not only drove Ferdinand out of central Hungary, but he also forced Ferdinand to agree to pay tribute for his lands in western Hungary.
John Sigismund Zápolya was also supported by his mother's father, Sigismund I the Old, King of Poland and Grand Duke of Lithuania. But in 1543 Sigismund I made a treaty with the Habsburgs and Poland became neutral; his son Sigismund II Augustus married Ferdinand's daughter, Archduchess Elisabeth of Austria.
The Eastern Hungarian Kingdom was ruled by John Sigismund Zápolya's mother Isabella Jagiellon, as regent, with Bishop Martinuzzi as the real power. But Isabella's hostile intrigues and threats from the Ottomans led Martinuzzi to switch round. In 1549, he agreed to support Ferdinand's claim, and Imperial armies marched into Transylvania. In the Treaty of Weissenburg (1551), Isabella agreed on behalf of John Sigismund Zápolya to abdicate as king of Hungary and to hand over the Holy Crown of Hungary and regalia. Thus Ferdinand acquired Royal Hungary and Transylvania; he recognised John Sigismund Zápolya as vassal Prince of Transylvania and betrothed one of his daughters to him. Meanwhile, Martinuzzi attempted to keep the Ottomans happy even after they responded by sending troops. Ferdinand's general, Castaldo, suspected Martinuzzi of treason and with Ferdinand's approval, had him killed.
Since Martinuzzi was by this time an archbishop and cardinal, this was a shocking act, and Pope Julius III excommunicated Castaldo and Ferdinand. Ferdinand sent the Pope a long accusation of treason against Martinuzzi in 87 articles, supported by 116 witnesses. The Pope exonerated Ferdinand and lifted the excommunications in 1555.
From 1548 to the end of the war, a Habsburg Spanish infantry tercio that had fought in the Schmalkaldic War was detached in Hungary to fight against John Zápolya's supporters on behalf of Ferdinand. Emperor Charles V sent Spanish troops to help his brother regularly from 1527 to 1553.
The war in Hungary continued. In the Ottoman campaign of 1552, two Ottoman armies took the eastern part of central Hungary. In 1554, Ferdinand sent Ogier Ghiselin de Busbecq to Constantinople to discuss a border treaty with Suleiman, but he could achieve nothing. In 1556, the Diet returned John Sigismund Zápolya to the eastern Hungarian throne, where he remained until 1570. De Busbecq returned to Constantinople in 1556 and succeeded on his second try.
The Austrian Habsburgs needed the economic power of Hungary for the Ottoman wars. During the Ottoman wars, the territory of the former Kingdom of Hungary shrank by around 70%. Despite these enormous territorial and demographic losses, the smaller, heavily war-torn Royal Hungary had remained economically more important to the Habsburg rulers than Austria or the Kingdom of Bohemia even at the end of the 16th century. Out of all his countries, the depleted Kingdom of Hungary was, at that time, Ferdinand's largest source of revenue.