First aid station directly behind front line, 1st Division, Nissy sur Bois, July 17, 1918 U.S. Signal Corps photo. A large group of soldiers, some sitting on benches, others lying on stretchers on the ground. During combat or training operations, military units may establish aid stations behind front lines to provide medical support to troops in the field. In United States military operations, these are most commonly referred to as Battalion Aid Stations; in Commonwealth countries, Regimental Aid Posts.