The Thompson was introduced into the US Marine Corps in 1922 for the purpose of guarding mail trains after a rash of robberies (which ceased after the Marines took over). The United State Military did not at first show much interest in acquiring this weapon, and so the manufacturer placed it on the open-market. Soon however the Thompson became popular with FBI Law enforcement officers combating gangsterism is Chicago in the 1920s and later, despite sanctions, a number of Thompsons found their way into the armouries of the Republican Army during the Spanish Civil War.
At the outbreak of the second World War the service model was the M1928A1 weighing some 10.45-lb.
1942 saw the introduction of the improved M1 and M1A1 versions which initially, were only issued to Divisional Scout , Military Police, and some 'Raider' units, although later , when sufficient became available they became available to other formations.
In April 1944 the Army ordered that the M1/M1A1s be withdrawn and be replaced with the new M3A1 'Grease-Gun'. The earlier models then being supplied to other counties.
The US Marine Corps retained some of their M1 Thompson in small numbers, although they were considered unsuitable for front line service since they sounded much like the Japanese 6.5mm light machine-guns.
A well known photograph of Mr Churchill holding a Thompson in 1940 does not reveal the fact that in 1940 there were only forty Thompson SMGs in the United Kingdom. These guns were quickly acquired by the newly formed Commandos who found them extremely useful in their cross channel raids.