Complex sentence: Discharged in 1917 with a respiratory disease after having received the Croix de guerre (or, war cross), he joined the surrealists, especially working with Louis Aragon, Gaston Bergery and his former schoolmate from the Lycée Carnot, Pierre Drieu La Rochelle.
Split into simpler sentences: Discharged in 1917 with a respiratory disease after having received the Croix de guerre (or, war cross), he joined the surrealists. There, he joined up with Louis Aragon, Gaston Bergery and his former schoolmate from the Lycée Carnot, Pierre Drieu La Rochelle.