The submachine gun was born in a World War 1 requirement seeking a high-volume weapon capable of clearing enemy forces from their trench positions. Such a weapon would have to be portable and fire (controllably) through a full-automatic action with lethal results at short-to-medium ranges. Semi-automatic pistols proved only a partial solution and machine guns of the period were simply too cumbersome to field in this fashion. As such, the "submachine gun" was born as a weapon class - first through the Italians and, later, refined into the classic form by the Germans. From then on, the class became an accepted portion of any military inventory.
The FIAT-Revelli Model 1915 - better known as the "Villar Perosa" - became (from a technical standpoint) the first submachine gun ever adopted by a national power for it fired a pistol cartridge through an automatic action and was of a highly portable design. However, the weapon was a far cry from the accepted "classic" submachine gun form, even by World War 1 standards, and this triumph was largely handed to the Germans for their Bergmann MP18 of 1918. Additionally, the Italian Army initially fielded their Villar Perosas in the light support role as a light machine gun (LMG), primarily issued to alpine forces for mountainous fighting. It was several years of warfare that the Italians finally realized what they had and rewrote tactics to suit the new weapon class. The Villar Perosa, therefore, became more of a mobile assault-minded weapon instead of a limited automatic support weapon.
It should be noted that the Villar Perosa is can be referred to by several names which tends to add to confusion - "FIAT" is the primary place of manufacture while Bethel Abiel "Revelli" di Beaumont is credited with its design. "Villar Perosa" is from the Turin-based headquarters of Officine Villar Perosa (OVP) where the original prototype was conceived and evaluated.