however, his tendency to run in his own privately entered alfa romeos did him no favours on the grand prix scene, and although a regular top-10 finisher in "grands épreuves" he never won a race. at the time, the german manufacturers mercedes-benz and auto union were the dominant force in grand prix racing, together with the french bugatti team. sommer turned to sports cars once more, and in 1936 he won the french grand prix with jean-pierre wimille, and the spa 24 hours endurance race with co-driver francesco severi. more wins came his way including at the "marseilles three hours" at miramas, the grand prix de tunisie and la turbie hill climb competition in 1938 and 1939 with alfa romeo 308 until the outbreak of world war ii, where he played an active part in the french resistance movement.