In order to rapidly equip a sufficient fighting force to counter the Axis threat, the Royal Small Arms Factory, Enfield, was commissioned to produce an alternative. American entry into the war at the end of 1941 placed an even bigger demand on the facilities making Thompsons. After the start of the war and to 1941 (and even later), the British purchased all the Thompson submachine guns they could from the United States, but these did not meet demand, and Thompsons were expensive, the M1928 costing $200 in 1939 (and still $70 in 1942), whereas a Sten would turn out to cost only $11. The army was forced to replace weapons lost during the evacuation from Dunkirk while expanding at the same time.

The Sten emerged while Britain was engaged in the Battle of Britain, facing invasion by Germany. Over four million Stens in various versions were made in the 1940s, making it the second most produced submachine gun of the Second World War, after the Soviet PPSh-41. Turpin, and 'En' for the Enfield factory. Sten is an acronym, from the names of the weapon's chief designers, Major Reginald V. The Sten is a select fire, blowback-operated weapon which mounts its magazine on the left. The Sten served as the basis for the Sterling submachine gun, which replaced the Sten in British service until the 1990s, when it, and all other submachine guns, were replaced by the SA80. They had a simple design and very low production cost, making them effective insurgency weapons for resistance groups, and they continue to see usage to this day by irregular military forces. The STEN (or Sten gun) is a family of British submachine guns chambered in 9×19mm which were used extensively by British and Commonwealth forces throughout World War II and the Korean War. Various underground resistance group factoriesģ.7–4.6 million (all variants, depending on source)

Sten mk ii folding stock