But the two great branches of the house of Hapsburg, the Spanish and the German, were actively arrayed on the same side; and the menace of Hapsburg supremacy was alarming.  About the time when Urban VIII. succeeded Gregory (1623), French policy, guided by Richelieu, was becoming definitely anti-Spanish, and organised a huge assault on the Hapsburgs, in conjunction with Protestants, though in France the Huguenots were quite subordinated.  This done, Richelieu found it politic to retire from the new combination, whereby a powerful impulse was given to Catholicism.

But Richelieu wished when free to combat the Hapsburgs, and Pope Urban favoured France, magnified himself as a temporal prince, and was anxious to check the Hapsburg or Austro-Spanish ascendancy.  The opportunity for alliance with France came, over the incidents connected with the succession of the French Duc de Nevers to Mantua, just when Richelieu had obtained complete predominance over the Huguenots.  Papal antagonism to the emperor was becoming obvious, while the emperor regarded himself as the true champion of the Faith, without much respect to the pope.

In this crisis the Catholic anti-imperialists turned to the only Protestant force which was not a beaten one—­Gustavus Adolphus of Sweden.  Dissatisfaction primarily with the absolutism at which the emperor and Wallenstein were aiming brought several of the hitherto imperialist allies over, and Ferdinand at the Diet of Ratisbon was forced to a change of attitude.  The victories of Gustavus brought new complications; Catholicism altogether was threatened.  The long course of the struggle which ensued need not be followed.  The peace of Westphalia, which ended it, proved that it was impossible for either combatant to effect a complete conquest; it set a decisive limit to the Catholic expansion, and to direct religious aggression.  The great spiritual contest had completed its operation.