The first submachine gun, which fires pistol cartridges and can be used by one soldier, was the MP18.1, invented by Theodor Bergmann. It was introduced in 1918 by the German Army as the primary weapon of the Stosstruppen (assault groups specialized in trench combat). More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.