He became involved in relief work as soon as the war broke out. As head of the American Relief Clearing House, he presided over the channelling of American contributions to France in currency and in kind. As chief representative of the American Red Cross in France from 1914-1917, he founded the Harjes Formation, a volunteer ambulance driver group which later merged with Richard Norton's American Volunteer Motor-Ambulance Corps to become the Norton-Harjes. When relief work was militarized under the American Red Cross in July 1917, Harjes stepped down and became chief liaison officer for the AEF with the French High Command.
Harjes, said to have introduced polo to France, had in 1923 lost his eldest daughter in a riding accident. Handicapped by a stiff leg from a war injury -- Harjes had told his wife he would stop playing polo altogether, but was killed in 1926, in a polo accident which occurred during the 'just one more game' he had promised his wife would conclude his polo career.