In the summer of 1634 the Hapsburg imperial army, led by Matthias Gallas, count von Campo (Gallas, Matthias, Count Von Campo, Duke Von Lucera), began to regain towns in southern Germany lost to Gustav II Adolf of Sweden and his allies two years earlier. Having taken Regensburg and Donauwörth, they laid siege to Nördlingen. There, a large Spanish army commanded by Ferdinand III, son of Holy Roman Emperor Ferdinand II, joined them from Italy. The combined forces prepared a heavily fortified position on a hill south of the town, which the army of Sweden and its Protestant allies, under the combined command of Gustav Karlsson Horn and Bernhard, duke of Saxe-Weimar (Bernhard of Saxe-Weimar), attacked. Although the Swedes gained an advantage on the first day of the battle, their forces were outnumbered and poorly led. The next day all attempts to dislodge the Habsburg army failed; some 12,000 Protestants fell, and 4,000 more (including Horn) were captured. Bernhard led the survivors northward, abandoning southern Germany to the victors. It was, claimed Spanish King Philip IV's chief minister, “the greatest victory of our times.”

Several German Protestants, led by Elector John George of Saxony (John George), now began to negotiate with the Habsburgs, concluding the Peace of Prague in May 1635, by which the elector and several other Lutheran (Lutheranism) rulers abandoned their Swedish alliance and joined forces with the Habsburgs. The German Calvinists (Calvinism) therefore turned to the only other foreign power capable of offering protection: France. Louis XIII of France immediately declared war on the Habsburgs and sent an army into Germany.