Agate volunteered in May 1915 at the age of thirty-seven for the Army Service Corps, and was posted to France. He had an arrangement to supply a series of open letters about his wartime experiences to Allan Monkhouse at "The Manchester Guardian". These were published in his first book, "L. of C." (Lines of Communication), of which a reviewer wrote, "Captain James E. Agate ranks as one of the first hundred thousand soldiers who have written a book about the war, but... one is sure there will be no other book like this one... It is our old friend 'J. E. A.' at his irritating best in khaki." Agate's fluency in French and knowledge of horses found him a successful job as a hay procurer, described in the first volume of his "Ego". His system of accounting for wartime hay purchases in a foreign land was eventually recognised by the War Office and made into an official handbook. Captain Agate's name was engraved on the Chapel-en-le-Frith War Memorial in Derbyshire. After "L of C", Agate published a book of essays on the theatre, "Buzz, Buzz!" (1918). In the same year, while still serving in France, Agate married Sidonie Joséphine Edmée Mourret-Castillon, daughter of a rich landowner. The marriage was short-lived, and after it broke up amicably, Agate's relationships were exclusively homosexual.