The tension in Europe preceding World War II weighed heavily on Brouha's mind and in February 1940, he and his wife left Belgium to join the French National Research Council in Paris; his laboratory in Liège was destroyed through bombardment during the German invasion of Belgium in May 1940. Guy La Chambre assigned him to the French Air Force, where he worked in the Laboratory of the Medico-Phsiological Services with physiologist André Mayer. Brouha's role was to test pilots for physical fitness and fatigue. But he soon answered Cannon's call to come to Harvard and he arrived in Boston in August 1940.