He spent the Second World War in the Free France forces: he embarked on 22 June 1940 at Saint-Jean-de-Luz for England and participated in the Battle of Dakar with Charles de Gaulle in September 1940.

He joined the Free French Forces in Chad, and the 2nd Armored Division in September 1944, and participated in the assault of Hitler's Eagle's nest at Berchtesgaden in 1945.

The seminars at the Institute were incomprehensible to Messiah who had only a high-school education in physics, and he became depressed to the extent that he considered abandoning his plans to become a physicist.

He returned to France and introduced the first general courses of quantum mechanics in France, at the University of Orsay and joined the newly created atomic energy agency, the Commissariat à l'énergie atomique (CEA) where he stayed until the end of his career.

Messiah collaborated with Oscar W. Greenberg on identical particle statistics other than bosons or fermions.