Leps joined the army on October 9, 1913. In 1915 he transferred to the French Air Service and served as an observer with Escadrille N67 until he was badly wounded on July 9. Having recovered from his wounds by the summer of 1916, Leps completed flight training at Amberieu and received his Pilot's Brevet on August 23, 1916. In December, he joined Escadrille N81 (later Spa81) at the front and scored his first victory by downing a German two-seater. During 1918, Leps shot down two more enemy aircraft and five balloons.

Chevalier de la Légion d'Honneur
"Young officer, courageous and ardent. Already wounded and cited in orders, conducted himself brilliantly on March 16, 1917, downing, during the course of the same flight, two enemy planes, one of which fell in our lines." Chevalier de la Légion d'Honneur citation, March 30, 1917