Upon being approached by Joachim Friedrich von Blumenthal at the Emperor Ferdinand's II. behest, he became one of the chief conspirators against Wallenstein, and after the tragedy of Eger was appointed to the command of the army which Wallenstein had formed and led. At the great battle of Nördlingen (23 August 1634) in which the army of Sweden was almost annihilated, Gallas commanded the victorious Imperial forces. His next command was in Lorraine, but even the Moselle valley had suffered so much from the ravages of war that his army perished of want.