The M1 helmet is an iconic piece of military equipment and is a combat helmet that was used by the United States' military forces from World War II until 1985. The M1 helmet was adopted by US military forces in 1941 to replace the outdated World War I-era British style Brodie helmet.
The M1 helmet is a combination of two layers of "one-size-fits-all" helmets—the first being an outer metal shell, sometimes called the "steel pot", and the second being a hard hat–type liner nestled inside it featuring an adjustable suspension and padding system. In addition, helmet covers, and netting could be applied by covering the steel shell with the extra material tucked inside the shell and secured by inserting the liner. Early liners were made from a mix of compressed paper fibres impregnated with phenolic resin that had an olive drab cotton twill fabric stretched over the outside. These were quickly discontinued in November 1942 as it was found that they degraded quickly in high heat and high humidity environments. They were replaced by evolving plastic liners which over the years changed in design and composition.
The outer shell was not to be worn by itself, although the design of the bowl-like shell led to some novel uses by the soldiers. When separated from the liner, the shell could be used as an entrenching tool, a hammer, washbasin, bucket, bowl, and as a seat. The shell was also sometimes used as a cooking pot, but the practice was discouraged by officers as the heat would make the metal alloy brittle. The inner liner can be worn by itself, providing protection similar to a construction hard hat, and was often worn in this fashion by military policemen and a variety of non-front-line military personnel