 In 1630, when Sweden joined the Thirty Years' War, Douglas followed the main army into Germany (where he joined the German literary society Der Fruchtbringenden Gesellschaft).Although a competent military commander, he only began to earn some notoriety in the aftermath of the Peace of Prague (1635).Then serving as a lieutenant colonel, Douglas had been tasked with the defence of Egeln in Saxony, and so was left very much within the territory of the newly hostile Elector Johan Georg who had defected to the Empire.