Richelieu took his measures promptly. In 1624 he helped to bring about a marriage alliance between Charles prince of Wales and Henrietta Maria, the sister of Louis XIII., by which he hoped to gain the assistance of England against Spain on the sea and in the Netherlands, while he struck at the Valtelline. An army of the mountaineers of the Grisons under French leadership drove the imperial troops from Chur, and the papal troops from the Valtelline, where they had replaced the Spaniards. Lesdiguières, at the head of a French force, marched to the assistance of Savoy against Genoa. But just at that time the Huguenots of La Rochelle flew to arms, and Richelieu, afraid of finding himself involved at once in war at home and abroad, came to terms with Spain at the treaty of Monzon, concluded in March 1626, by which the Valtelline was to remain under the control of the Grisons. [Sidenote: =The Mantuan succession, 1627.=] For the next three years the whole energies of Richelieu and of France were engaged in the reduction of La Rochelle, and in the war with England, which followed hard upon, and indeed sprung out of, the marriage treaty of 1624. In 1629 he was once more at liberty to turn his attention to Italian affairs. In 1627 the duke of Mantua and Montferrat had died. His nearest heir was a Frenchman, the duke of Nevers. But the Emperor, at the instigation of Spain, not wishing to have a French prince so near the Milanese, determined to sequester the territory on the pretext of a disputed succession. Spanish troops at once overran both Mantua and Montferrat, and driving the duke of Nevers into Casale besieged him there. The Italian princes, however, were not inclined to submit without protest to this exercise by the Emperor of obsolete and doubtful rights. The Pope (Urban VIII.), who was strongly French in sympathy, combined with Venice to ask the assistance of France, and in January 1629 Louis and Richelieu crossed the Mont Genèvre at the head of a large army, captured Susa, relieved Casale, and forced the duke of Savoy to make peace. Again, however, a rebellion of the Huguenots obliged Louis to draw back in the hour of victory (March 1629), and in the summer of that year fresh troops, set free by the imperialist successes in Germany, invaded Italy under Spinola and formed the sieges of Mantua and Casale. In spite of the most strenuous efforts of Louis himself, who crossed the Alps at the head of the French armies in the winter of 1629–30, the combined forces of Spain and the Empire were too strong to be dislodged from Mantua or Montferrat. [Sidenote: =Peace of Cherasco, 1631.=] But the invasion of Germany by Gustavus Adolphus, promoted by France and even by the Pope, made the Emperor anxious for peace, and through the diplomatic skill of the papal agent, Giulio Mazzarini--afterwards to become so celebrated in French history--a truce was arranged, which afterwards ripened into the definitive peace of Cherasco (April 26th, 1631).