The Thompson Submachine Gun was developed by General John T. Thompson who originally envisioned an auto rifle (semi-automatic rifle) to replace the bolt action service rifles then in use. While searching for a way to allow such a weapon to operate safely without the complexity of a recoil or gas operated mechanism, Thompson came across a patent issued to John Bell Blish in 1915 based on adhesion of inclined metal surfaces under pressure. Thompson found a financial backer, Thomas F. Ryan, and started the Auto-Ordnance Corporation in 1916 for the purpose of developing his auto rifle. The principal designers were Theodore H. Eickhoff, Oscar V. Payne, and George E. Goll. By late 1917, the limits of the Blish Principle were discovered: rather than working as a locked breech, it functioned as a friction-delayed blowback action. It was found that the only cartridge currently in U.S. service suitable for use with the lock was the .45 ACP round. Thompson then envisioned a "one-man, hand-held machine gun" in .45 ACP as a "trench broom" for use in the on-going trench warfare of World War I. Payne designed the gun itself and its stick and drum magazines. The project was then titled "Annihilator I", and by 1918, most of the design issues had been resolved. However, the war ended before prototypes could be shipped to Europe.
The Thompson first entered production as the M1921. It was available to civilians, though its high price resulted in few sales. (A Thompson M1921 with one Type XX 20 shot "stick" magazine was priced at $200.00 when a Ford automobile sold for $400.00.) M1921 Thompsons were first sold in small quantities to the U.S. Post Office (to protect the mail from a spate of robberies), followed by several police departments in the United States and minor international sales to various armies and constabulary forces, chiefly in Central and South America. The U.S. Post Office also gave Thompsons to the U.S. Marine Corps in 1922 when Marines were assigned to protect against mail robberies, with the Marines putting them to use in the Banana Wars and in China. It was popular with the Marines as a point-defense weapon for countering ambush by Nicaraguan guerrillas and led to the organisation of 4 man fire teams with as much firepower as a 9 man rifle squad. The major complaints against the Thompson were its weight, inaccuracy at ranges over 50 yards, and its lack of penetrating power, despite the powerful round it used.
In 1938, the Thompson submachine gun was adopted by the U.S. military, serving during World War II and beyond.
By the time of the Korean War, the Thompson had seen much use by the U.S. and South Korean Military, even though Thompson will have been replaced in production by the M3 and M3A1. Many Thompsons were distributed to Chinese armed forces as military aid before the fall of Chiang Kai-Shek's government to Mao Zedong's Communist forces in 1949. During the Korean War, American troops were surprised to encounter Chinese Communist troops heavily armed with Thompsons, especially during surprise night assaults. The gun's ability to deliver large quantities of short-range automatic assault fire proved very useful in both defense and assault during the early part of the conflict. Many of these weapons were captured and placed into service with American soldiers and Marines for the balance of the war.