Hermle was assigned to the 74th Company, 1st Battalion of the 6th Marine Regiment, and sent to the France. His company was sent to the frontline in the Toulon Sector near Verdun in March 1918. A month later, the Germans used combat gas and Hermle was wounded by its effects. After recovery, Hermle was promoted to the rank of first lieutenant and appointed a platoon leader in the 74th Company in the 1st Battalion. He participated in the Battle of Saint-Mihiel and was decorated with the Silver Star for heroism in action near the town of Thiaucourt. He continued to command his platoon, and at the beginning of the October 1918, he led a successful assault up the hill at the Blanc Mont and was decorated with his second Silver Star.