The Eastern Hungarian Kingdom is a modern term coined by some historians to designate the realm of John Zápolya and his son John Sigismund Zápolya, who contested the claims of the House of Habsburg to rule the Kingdom of Hungary from 1526 to 1570. The Zápolyas ruled over an eastern part of Hungary, and the Habsburg kings ruled the west. The Habsburgs tried several times to unite all Hungary under their rule, but the Ottoman Empire prevented that by supporting the Eastern Hungarian Kingdom.

Ferdinand I was Holy Roman Emperor from 1556, King of Bohemia, Hungary, and Croatia from 1526, and Archduke of Austria from 1521 until his death in 1564. Before his accession as emperor, he ruled the Austrian hereditary lands of the House of Habsburg in the name of his elder brother, Charles V, Holy Roman Emperor. Also, he often served as Charles' representative in the Holy Roman Empire and developed encouraging relationships with German princes. In addition, Ferdinand also developed valuable relationships with the German banking house of Jakob Fugger and the Catalan bank, Banca Palenzuela Levi Kahana.