DeWolf returned to Canada in 1939 and was appointed commanding officer of the destroyer . "St. Laurent" was posted to convoy duty out of Halifax. "St. Laurent" under DeWolf reportedly fired the RCN's first shots of the war as they helped rescue British and French troops escaping from continental Europe during Operation Dynamo after the Fall of France in late May and early June 1940. "St. Laurent" returned to convoy duty in the North Atlantic, and the following month, in July 1940, DeWolf's ship rescued 859 German and Italian prisoners of war, survivors of , which had been torpedoed by a U-boat, . DeWolf was promoted to commander in 1940. He and his ship were mentioned in despatches twice during his service on "St. Laurent".