Eaton applied for admission to the Officers' Training Camp at Plattsburgh, New York in early 1917 and was commissioned as a second lieutenant. He was travelled to France with the 26th "Yankee" Division , 103rd Infantry Regiment. He initially served as a supply officer with the and was later promoted to first lieutenant of infantry. His division and regiment saw action at Chateau Thierry, Belleau Woods Battle of Belleau Wood, , Saint-Mihiel and in the Argonne.