Isabella Jagiellon (Hungarian: Izabella királyné; Polish: Izabela Jagiellonka; 18 January 1519 – 15 September 1559) was the queen consort of 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.

Her husband's death sparked renewed hostilities but Sultan Suleiman established her as a regent of the eastern regions of the medieval Kingdom of Hungary on behalf of her infant son.

[1] Ferdinand, however, never renounced his claims to reunite Hungary and conspired with Bishop George Martinuzzi who forced Isabella to abdicate in 1551.

Sultan Suleiman retaliated and threatened to invade Hungary in 1555–56 forcing nobles to invite Isabella back to Transylvania.

Born in Kraków on 18 January 1519,[3] Isabella was the oldest child of King Sigismund I the Old and his Italian wife Bona Sforza.

She hoped that King of France would install his son and Isabella in the Duchy of Milan which Bona claimed as her inheritance.

[6] In 1524, Hieronymus Łaski negotiated an anti-Turkish alliance with the French; among the provisions was marriage of Isabella and Henry, second son of Francis I.

Bona then proposed Federico II Gonzaga, Duke of Mantua, but he chose Margaret Paleologa as she brought March of Montferrat as her inheritance.

In 1530, he proposed Archduke Maximilian, eldest son of King Ferdinand, but they refused as Isabella was eight years older than the groom.

[citation needed] In April 1538, just two months after the Treaty of Nagyvárad, Bishop Stjepan Brodarić arrived at Kraków to negotiate timing and conditions for the wedding of Isabella and John Zápolya.

Therefore, he annexed most of Hungary[14] and only Transylvania and lands east of the Tisza river (known as Temesköz) were given to John Sigismund and Isabella as his guardian.

[19] She faced multiple political challenges: keeping peace with the Ottomans, containing Martinuzzi's ambitions, and blocking Ferdinand's attempts at taking over Transylvania.

In December 1541, perhaps exhausted by her circumstances, Isabella signed an agreement with Ferdinand: she would abandon Transylvania and take up residence in the Szepes County.

[14] However, when Ottomans attacked again, Ferdinand did not have enough military power to defend his territories (see Siege of Esztergom (1543))[12] and the agreement remained forgotten.

Poland proposed to solve the issue with another marriage – Isabella would marry widowed Charles V, Holy Roman Emperor, Ferdinand, or his eldest son Maximilian.

She did not receive help from her brother Sigismund II Augustus as he concluded an alliance with Ferdinand so that he could quell opposition of Polish nobility to his marriage to Barbara Radziwiłł.

According to Marcin Bielski, the cross atop of the crown was broken off and kept by John Sigismund who hoped one day to reunite the pieces.

According to a legend, when Isabella stopped to have a rest at the gates of Meszes, a border town, she cut the abbreviation of her motto into the bark of an old oak tree: SFV – Sic fata volunt ("It is the will of fate").

The buildings where she was to live were unsuitable for habitation, the income was only half of what the treaty provided, and Ferdinand did not hurry to pay the agreed cash sums.

[28] Isabella received invitations both from Sultan Suleiman, who sent an envoy to Poland, and local nobles to return to Hungary, but she delayed.

[26] Her brother King Sigismund II Augustus, afraid of a Habsburg–Russian alliance in the series of the Muscovite–Lithuanian Wars, married Catherine of Austria, daughter of Ferdinand.

[26] Isabella and her mother Bona Sforza demanded that Ferdinand fulfilled his obligations according to the Treaty of Weissenburg, but he did not have financial resources to comply and asked for concessions.