later, they won a decisive victory at Nördlingen which destroyed Swedish power in Southern Germany and led to the defection of their German allies, who now sought to make peace with the Emperor. Phase II: French intervention from 1635 to 1648 By triggering direct French intervention, Nördlingen expanded the conflict rather than ending it. Richelieu provided the Swedes with new subsidies, hired mercenaries led by Bernard of Saxe-Weimar for an offensive in the Rhineland and in May 1635 formally declared war on Spain. A few days later, the German states and Ferdinand agreed the Peace of Prague; in return for withdrawing the Edict of Restitution, the Heilbronn and Catholic Leagues were dissolved and replaced by a single Imperial army, although Saxony and Bavaria retained control of their own forces. This is generally seen as the point when the war ceased to be a primarily inter-German religious conflict. In March 1635, French soldiers entered the Valtellina, cutting the link between Spanish controlled Milan and the Empire. In May, their main army of 35,000 invaded the Spanish Netherlands, but withdrew in July after suffering 17,000 casualties. The Spanish responded in 1636 with an offensive that reached Corbie in Northern France, causing panic in Paris before they had to retreat. In March 1636, France joined the war in support of Sweden, whose loss of most of the territories gained by Gustavus and their taxes made it increasingly reliant on French financing. Although most of their German allies defected following the Peace of Prague, the Swedes under Johan Banér defeated an Imperial force at Wittstock on 4 October, re-establishing their predominance in North-East Germany. Ferdinand II died in February 1637 and was succeeded by his son Ferdinand III, who faced a deteriorating military position. Although Matthias Gallas and the main Imperial army had forced Banér back to the Baltic, in March 1638, Bernard destroyed an Imperial army at Rheinfelden. His capture of Breisach in December secured French control of Alsace and severed the Spanish Road, forcing Gallas to divert resources there. Although von Hatzfeldt defeated a combined Swedish-German force at Vlotho in October, Gallas withdrew from the Baltic due to lack of supplies, while his army partially dissolved in the process. In April 1639, Banér defeated the Saxons at Chemnitz, then entered Bohemia in May. To retrieve the situation, Ferdinand diverted Piccolomini's army from Thionville, ending direct military cooperation between Austria and Spain. This