The Treaty of Nagyvárad (or Treaty of Grosswardein) was a secret peace agreement between Emperor Ferdinand I and John Szapolyai, rival claimants to the Kingdom of Hungary, signed in Grosswardein / Várad (modern-day Oradea, Romania) on February 24, 1538.
[1] In the treaty, they divided Hungary between them according to the actual possession.
Ferdinand recognized Zápolya as John I, King of Hungary and ruler of two-thirds of the Kingdom, while Zápolya conceded the rule of Ferdinand over western Hungary, and recognized him as heir to the Hungarian throne, since Zápolya was childless.
Isabella asked Ottoman Sultan Suleyman I to help in the fight with Ferdinand and his successors that ensued, only to see Suleiman to prevail, declare John II a king, and placing himself as a regent.
A large portion of Hungary became essentially a Turkish province, complete with Ottoman governor and garrison in Buda.