A purely religious reason also made him Richelieu's ally against the Habsburgs. He had a dream of arousing Europe to another crusade against the Ottoman Empire and believed that the Habsburgs were an obstacle to making this possible. For Richelieu, he manoeuvered at the Diet of Regensburg (1630) to thwart the aggression of the Habsburg emperor, and then recommended the intervention of Gustavus Adolphus, and the Protestant armies, thereby maintaining a balance of power.[citation needed]