The M15 Adrian helmet (French: Casque Adrian) was a combat helmet issued to the French Army during World War I. The first standard helmet of the French Army, it was designed when millions of French troops were engaged in trench warfare and head wounds became a significant proportion of battlefield casualties. Introduced in 1915, it served as a basic helmet of many armies well into 1930s. Initially issued to infantry, its modified versions were also issued to cavalry and tank crews. Its subsequent version, the M26, was adopted later and used during World War II.
At the outbreak of World War I, the French army was equipped with the standard kepi cap, providing no protection against enemy fire, direct or indirect. Early stages of trench warfare proved that even basic protection of the head could result in a significantly smaller mortality rate among the front-line soldiers. Consequently, the French staff ordered development of a metal helmet that could protect the soldier from the shrapnel of exploding artillery shells. Since soldiers in trenches were also vulnerable to shrapnel exploding above their heads, a deflector crest was added along the helmet's axis. Contrary to common misconception, the M15 helmet was not designed to protect the wearer from direct impact by rifle or machine gun bullets. The resulting headgear was credited to Intendant-General August-Louis Adrian.
The helmet adopted by the army was made of mild steel and weighed only 0.765 kg (1 lb.11oz.), which made it lighter than the contemporary British Brodie helmet and the German Stahlhelm, although it also delivered less protection against shrapnel and bullets. By the end of World War I, it had been issued to almost all infantry units fighting with the French army. It was also used by some of the American divisions fighting in France and the Polish forces of Haller's Blue Army.
The helmet proved to be fairly efficient against shrapnel, cheap, and easy to be manufactured. As a consequence, more than three million Adrians were produced, and they were widely adopted by other countries including Belgium, Greece, Italy (including license-built versions), Japan, Luxembourg, Mexico, Morocco, Peru, Poland, Romania, Russia, Serbia, Siam, U.S.A., U.S.S.R., and Yugoslavia. It was also adopted briefly by the National Army in the Irish Free State. Each of these nations added its own insignia to the front of the helmet.
Because the new steel helmets offered little actual protection against bullets, some were reportedly among the first pieces of equipment being abandoned by the soldiers on the battlefield. It was also discovered that the badge placed on the front of helmets impaired the helmet's durability, which made several armies remove their national insignia altogether.
In the French army, the Adrian helmet was standard military issue until after World War II, and was also used by the French police up to the 1970s. In other countries the Adrian-type helmets were also in use with the fire fighting units, railway guards or marine infantry. Adrian helmets are still prized by collectors today.