He spent time in Chile and then recrossed the Atlantic to Africa. War had broken out, from the French perspective, in September 1939, which was followed by the German invasion and occupation of northern France in May/June 1940. The Germans never conquered the French imperial territories in Africa, however, and on 26 June 1942 Mercier-Vega signed up as a volunteer with the Free France (government in exile) organisation which was commanded, mostly from London, by Charles de Gaulle. By December 1942 he was in Beirut where, between April and December 1945, he was seconded to the Information Service (Radio Levant) of the Free French government. He ended the war with the rank of Adjutant. He left the region, embarking from Alexandria, in October 1945, landing at Marseilles on 11 November. He was demobilised in Paris on 6 December 1945.
