Bloch joined the army on 7 September 1914. After receiving a Pilot's Brevet on 12 October 1915, he was posted to Escadrille N3 in January 1916 but scored no victories with this unit. On 25 May 1916, he was reassigned to N62 where he was severely wounded while scoring his third victory on 3 July. After returning to duty, he downed two more observation balloons in the fall of 1916. Transferred to the Russian Front on 23 March 1917, he was badly injured in a flying accident on 8 May and was in and out of hospitals for months. On 10 September 1918, he served with the French Mission to the United States before returning to France on 1 March 1919. During the war, Bloch accumulated 405 hours of flight time in combat.
"Remarkable pilot having a high point of perception and duty. His bravery proven to all, he has distinguished himself during the course of combats, by attacks on balloons on 26 and 29 June, 1 and 3 July 1916, returning each time with his plane riddled by bullets. On 3 July 1916, charged with destruction of a balloon, he accomplished his mission after having received two severe wounds." Médaille Militaire citation, 14 August 1916