The M1 helmet is a combat helmet that was used by the United States military from World War II until 1985. For over forty years the M1 became an icon of the American military, with its design inspiring other militaries around the world.
It is estimated that 22 million M1 helmets were produced during WW2, and approximately 1 million between 1966-1967 during the Vietnam War.
The bulk of the helmet is the shell and is constructed from a single piece of pressed hadfield manganese steel. The shape, liner, webbing and chin strap went through various changes to accodomate evolving warfare.
During Vietnam, soldiers often opted to wear the chinstrap unfastened, instead looping it around the back of the helmet. This practice arose for two reasons: firstly, because during unarmed combat it was assumed the enemy could attack from behind, easily reach over the helmet, grab the visor and pull back. If the chinstrap was fastened under the chin, the head would be snapped back causing the wearer to lose balance leaving his throat exposed to a knife attack. Secondly, soldiers believed that the concussive force of a nearby explosion could cause the helmet to break their neck. This wasn't without justification and a replacement buckle, T-1 Pressure Release Buckle, was designed to break over 15lbs of force.
 
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