Henry Gibson recalled that Putnam approached him at his tent and said, "I am going across the seas to get in the big fight." He did indeed, by way of a cattle boat, and joined the French Foreign Legion on May 31, 1917. He was transferred to the air service and trained at Avord. Putnam was assigned to Escadrille SPA 94 on 12 December 1917, and was posted to SPA 156 on 7 February 1918. While with the latter unit, he shot down four planes before being transferred to SPA 38 on 1 June 1918, where he claimed two more planes shot down. He was discharged in June 1918.