The submachine gun is an automatic or selective-fired shoulder weapon that fires pistol-caliber ammunition. The concept of submachine gun dates back to World War One; the trench warfare of this war required effective and compact weapons for short-range fighting in trenches; additionally, a lightweight and maneuverable fully automatic weapon was desirable to complement light machine guns in both defensive and offensive scenarios, to cover last 200 meters of assault on enemy positions. The first weapon which can be considered to some extent as the world's first submachine gun was the Italian Villar-Perosa, which was a twin-barreled automatic weapon that fired 9mm Glisenti pistol ammunition from top-mounted box magazines. It was compact, but its primary tactical role was of short-range machine gun; therefore it was usually fired from some sort of mount, and fitted with machine-gun type spade grips instead of more conventional rifle-type stock.
German police officer fires Schmeisser MP.18,I submachine gun (circa 1920).
The first true submachine gun was the Bergmann / Schmeisser MP.18,I, which saw some action during closing days of the Great war. This was a shoulder-fired weapon, that set the basic pattern for all following weapons of its class. The inter-war decades produced a significant number of submachine guns, but the tactical niche for these weapons was still unclear for many military experts. It was the Grand Chako war, the Spanish Civil war and Russo-Finnish Winter war of 1940 that proved the viability of submachine guns as general-issue weapons for fighting troops. Nevertheless, regardless of the large number of available models, by the start of World War Two in most armies submachine guns were relegated to secondary role. For example, the very technically advanced Wehrmacht (Hitler's army) issued MP-38 and MP-40 submachine guns to infantry troops in proportion of about one SMG per ten bolt action rifles. It was the Red (Soviet) army which issued PPSh-41 submachine guns as primary infantry weapons to entire companies and battalions. Despite the success of several new submachine guns, developed during the WW2, this war marked the start of decline of submachine guns as primary infantry weapons. The appearance of assault rifle, which, while being only slightly heavier than most SMGs, had much longer effective range, put an abrupt end to infantry use of submachine guns in Soviet army. On the other hand, the NATO countries still issued 9mm submachine guns to many non-infantry units and certain soldiers in infantry (i.e. scouts, machine gun and mortar crews etc) to complement relatively large and heavy semi-automatic or fully-automatic rifles firing powerful 7,62×51 NATO ammunition. The appearance (and wide distribution) of small-caliber assault rifles marked the final phase of history of submachine gun as general-issue infantry weapon.
American soldier fires Thompson submachine gun (WW2).
Despite of all said above, it must be noted that submachine guns still posses several qualities that are very useful in certain military scenarios. For example, submachine guns can be easily silenced, making them very useful for various special operations forces.
The police and security use of submachine guns, on the other hand, has been greatly increased during last 30 or 40 years. Proliferation of international terrorism, drugs trafficking, gang crime and other violent crimes forced many police forces to adopt a variety of submachine guns for special police teams. Compact submachine guns, which appeared during 1960's and 1970's, such as Micro-Uzi or HK MP5k, were quickly adopted by various VIP protection teams that favored compact size combined with massive short-range firepower of such guns. Of cause, the other side of the law also saw benefits of submachine guns; for example, more than few gangsters, outlaws and terrorists used various submachine guns, starting with "Chicago typewriter" (Thompson submachine gun) and up to Czechoslovak Scorpion or Croatian Agram 2000.