Following the conclusion of the war in Finland in March 1940, he returned home and resumed his life as a student at Leiden. He was there when Battle of the Netherlands was fought after the German invasion on 10 May 1940. Roelfzema joined the Dutch army reserve but the Netherlands was overrun by the German army in five days, bringing the war to a close before he was deployed. Queen Wilhelmina of the Netherlands fled to London to establish the government in exile there.