Who invented the sub-machine gun?

Answer 1

Gen. William Thompson. in 1919. The Thompson SMG became known as the Tommy Gun.

Before that, it was the German Bergmann MP-18, invented in 1918

Answer 2

The first practical sub machine gun used in combat was the Bergman MP18, from Germany. As the name indicates, it dates from 1918. See the link below for more information.