During the First World War, millions of horses served on the Western Front in all armies, usually in the cavalry, pulling guns or hauling supplies.
Horses could withstand higher concentrations of poison gas than humans, but chemicals could still damage their lungs, or burn their legs and hooves. Gas masks, like the one seen in this photograph, were developed to protect horses from poison gases. Often overworked and underfed, horses died in shocking numbers throughout the war.
