Oñate treaty

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The Oñate treaty was a secret agreement made in 1617 between the Austrian and Spanish branches of the Habsburg family. It was negotiated by Iñigo de Oñate, the Spanish ambassador in Vienna.

Under the deal, Ferdinand II, the Austrian archduke, would be the only Habsburg allowed to try to become king of Bohemia and Hungary after Matthias died childless. In return, Philip III of Spain would keep undisputed control over Habsburg lands along the "Spanish Road"—a chain of territories from northern Italy to the Spanish Netherlands—to support Spain’s wars in the Netherlands.

Ferdinand II promised Philip III important territorial gains: Finale Ligure and Piombino (already controlled by Spain) and control of Ortenau and Alsace. The agreement was kept secret from Cardinal Melchior Klesl, Matthias’s main adviser, who hoped to keep the empire united and ease religious tensions.

The treaty helped Ferdinand II advance his position, and he won the Bohemian and Hungarian crowns in 1617–1618. After Matthias died in 1619, the Bohemian estates chose Frederick V as a Protestant rival, while Ferdinand II was elected Holy Roman Emperor in 1619. With Spanish support, this led to the Battle of White Mountain in 1620, strengthening Ferdinand’s rule.