When World War II broke out, Moncel went to Europe with the Royal Canadian Regiment (RCR). In May 1940, he was ordered along with the RCR to France to reinforce the British Expeditionary Force facing the German "blitzkrieg". Soon however, Lieutenant Moncel, who commanded a Bren gun carrier platoon, was ordered to retreat to the French coast. When being evacuated, he was ordered to destroy his equipment to save it from German hands, but Moncel, with his cool judgement, managed to evacuate the Bren gun carriers. This act caught the eyes of his superiors and he was promoted to captain.