Treaty of Nagyvárad

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The Treaty of Nagyvárad (also called the Treaty of Grosswardein) was a secret peace deal signed on February 24, 1538, in Grosswardein/Várad (modern Oradea, Romania) between Emperor Ferdinand I and John Zápolya, competing kings of Hungary. The agreement split Hungary based on who controlled it at the time: Zápolya would rule two-thirds of the kingdom as John I, while Ferdinand would rule western Hungary and be recognized as the heir to the throne since Zápolya had no children.

In 1540, just before Zápolya died, his wife Isabella gave birth to a son, John Sigismund Zápolya. This undermined the deal, and the son was later elected king by the Hungarian nobility as John II.

Isabella then asked Ottoman sultan Suleiman I for help against Ferdinand and his successors. Suleiman supported John II, declared him king, and took a regent role for himself. A large part of Hungary effectively became an Ottoman province, with an Ottoman governor and garrison in Buda.