At the outbreak of World War II in September 1939, Kopp joined the French Foreign Legion as a corporal and fought in the Battle of France in May–June 1940, barely surviving the Blitzkrieg that many of his comrades did not. Kopp was severely wounded and made prisoner but escaped from a military hospital and was able to rejoin his unit, now a defeated army, just outside of Marseilles in August. The following month, Kopp was transferred to Foreign Legion headquarters in Algeria, where he was invalided out of the army and was awarded an 80% pension.