[6] During the Thirty Years' War, the Spanish sent an army from Brussels under Ambrosio Spinola to support the Emperor, and, as the Spanish ambassador in Vienna, don Íñigo persuaded Protestant Saxony to intervene against Bohemia in exchange for control over Lusatia.
The Saxons invaded, and the Spanish army in the West prevented the Protestant Union's forces from assisting.
As Spanish Ambassador, he openly took sides against Albrecht von Wallenstein, to whose downfall he contributed decisively.
Ferdinand II decisively defeated Frederick V at the Battle of White Mountain, near Prague on 8 November 1620.
In Spain he had ambitions to follow the Duke of Lerma as valido, but the King's favourite and Íñigo's adversary was chosen, the Count-Duke of Olivares.