Authored By Dan Alex (Updated: 7/23/2016):
General Thompson, son to an Army Lieutenant Colonel, took to heart the hard lessons being learned in "trench warfare" tactics of World War 1. The infantryman was generally issued a long bolt-action rifle (that is, every round had to be manually prepped into the chamber before firing through use of a bolt lever) with attached bayonet. While the rifle proved worthy of ranged warfare, it was of little value in the confined spaces of the trench networks dotting the European countrysides. When in close quarters, the bayonet, attached along the underside these long rifles, was hardly the answer to the common soldier and a better solution was in order. Additionally, World War 1 was a sort of battlefield where the appearance of even a single machine gun - and its inherent firepower, not to mention the psychological implications - could very well make-or-break a given offensive.
As such, Thompson - with an education in engineering and artillery and work experience in the US Army's Ordnance Department - envisioned a "hand-held" version of a machine gun - a portable "trench-sweeping" system capable of operation by a single soldier and optimized for use in intimate quarters. The challenge lay in devising a system that was relatively simple, safe to use and made to fit into the hands of the standard infantryman.
In 1915, John Bell Blish received his patent for a friction delayed blowback firing action to which Thompson happened across. The method of fire basically called for the slowing down of the breech by frictional forces accomplished by two obliquely-angled blocks sliding over one another. This operation could effectively bring down the cyclic rate-of-fire of a given weapon system and seem to be what Thompson had been searching for. Thomas Fortune Ryan provided the funding and Thompson began his Auto-Ordnance Corporation in 1916. Design followed under the name of "Annihilator I" and the American military .45 ACP round was selected - this essentially being the only cartridge round suitable for use in the type of Blish firing action being used.
The design was completed in 1918 but missed out on potential orders with the close of World War 1. Had it been produced and delivered in time, it is interesting to envision the Allied soldier perusing the deadly trenches of Europe with his Thompson in hand. In post-war America, the Annihilator name was now dropped in favor of "Thompson Submachine Gun". Most further development would now occur in the commercial market with the military showing little interest in such a system.
The German MP18
It should be noted that the first practical submachine gun was actually of German origin and fielded in World War 1 as the MP18 ("Machine Pistol" and noted by the initial year of service being 1918). It was a development of Hugo Schmeisser in 1916 and produced by Theodor Bergmann under Bergmann Waffenfabrik, serving the German Army and others from 1918 through 1945. It operated from an open bolt/blowback principle and made use of 9x19mm Parabellum with a rate-of-fire equal to 500 rounds-per-minute. The implication of this weapon in the war forced the Versailles treaty to restrict future development of the weapon type - though the foundation of the "submachine gun" class had already been laid only to be perfected by the time of World War 2. In essence, this shows how many other forces in the world were also in the same line of thinking at the time - to devise a capable portable system fielding the firepower of a machine gun.
M1921 - the First Thompson
The M1921 became the first Thompson model to be produced and this only found a few homes in the civilian, government and police markets, many put off by the types high costs. Despite some security and military use, the Thompson soon found popularity amongst the gangsters of the Prohibition era and, equally, in the police forces charged with taking them down.
Second World War
With rumblings of World War again in the air, overseas operators such as China purchased and operated the weapon against Japanese forces. The US military officially took on the weapon in 1938. While the M1928A1 was the weapon on hand by the time of the Japanese attack on Pearl Harbor, a simplified version was desperately needed to fill the needs of the US military. As such, the simplified M1 and M1A1 soon appeared.
Combat actions led the weapon to be used in both European and Pacific battlegrounds - proving more effective in the former. European allies were surprised to see Germans making large-scale use of submachine guns themselves and quickly clamored for a similar weapon with help from the United States. Orders began coming in and, while Yugoslavian and French orders were diverted with their respective falls, the United Kingdom became one of the principle early operators of the Thompson.
Patrols in the Pacific soon made use of the more powerful BAR light machine gun it place of their Thompsons. The high rate-of-fire offered by the little machine gun and man-stopping qualities inherent in the .45 ACP slug proved a godsend to troops in close-quarters though this was offset to an extent by the weapons heavy weight and unique ammunition. Despite its issuing to Soviet troops via Lend-Lease, the lack of available .45 ammunition in the country led to its limited use along the East Front. The weapon saw use in the hands of American, British, Australian and Canadian forces. By late 1944, the M3 "Grease Gun" series - a much cheaper production alternative - was replacing the Thompson in service - though many still appreciated the qualities of the their Thompsons instead.
The Thompson appeared in the Korean War and, perhaps more amazingly, in the Vietnam War. It was also reported to be in use during the Bosnian War (1992 - 1995).
The Thompson maintained a unique profile never seen before and never repeated again. The body bore a boxy type appearance with the rear sight dominating the top rear end. The pistol group was mounted midway under the body with the "stick" type magazine just forward of that. The barrel (of the M1) was fitted with a forward sight at the barrel end and the underside held a horizontal foregrip of wood. Wood also complimented the pistol grip and the fixed buttstock. A sling could make use of the lugs located at the buttstock base and the foregrip base. The ammunition ejection port was to the right side on top of the weapon just above the magazine feed. The rear sight of a flip-up adjustable sight.