The MP 18 primarily served in final stages of World War I in 1918, especially in the so-called Kaiserschlacht offensive. At least 5,000 MP 18.1s were built and used during World War I, based upon observed serial number ranges of captured weapons; however, it is possible that up to 10,000 were built for the war. Though production was outlawed by the Treaty of Versailles, manufacture continued in secret into the early 1920s, as the final production total (again, based upon observed serial numbers) ended at around 35,000.
The MP 18 proved to be an excellent weapon. Its concept was well-proven in trench fighting. The basic design directly influenced later submachine gun designs and showed its superiority over the regular infantry rifle in urban, mobile, and guerrilla warfare. The MP 18 served with German police and paramilitary forces after the end of the war. It was widely used in combat by the Freikorps Von Epp against the Spartacus League in Bavaria and by other Freikorps in Berlin, where its efficiency in urban combat was demonstrated. Since the Treaty of Versailles prohibited the use and manufacture of pistols with a barrel over four inches (102 mm) and/or a magazine capacity over eight rounds, all Trommel Magazines TM 08 were destroyed.
All the limited conflicts between 1920 and 1940 saw an increasing use of this new class of weapons, first in South America during the Chaco War, then in Europe during the Spanish Civil War, and in China during the Japanese invasion, where its use by well-trained Chinese troops was costly for the invaders as in the Battle of Shanghai, where fierce street fights prefigured World War II urban combat of Stalingrad, Warsaw, Vienna and Berlin.
Since the treaty allowed the Weimar Republic to keep a small quantity of submachine guns for police use, a few hundred MP 18.1s were modified to accept Schmeisser's original 20-round magazine design. This modification, conducted by Haenel Waffenfabrik, required removal of the existing magazine well collar, and replacement with a different one. These weapons were overstamped with the date "1920" on the receiver and magazine well to show they were legitimate weapons owned by the Weimar Republic and not war bringbacks or clandestine weapons.
Bergmann sold the license of the MP 18. 1 to SIG Switzerland; the Swiss made model was known as SIG Bergmann 1920. It existed in .30 Luger, 9mm Parabellum and 7.63 mm Mauser. The Bergmann MP 18.1 represents a milestone both in terms of armament technology and warfare tactics. It opened the way for a whole new class of weapons and triggered the research for lighter automatic firearms to be used by mobile troops. Its first direct competitors did not see service in World War I, but most of them saw use in all the limited conflicts taking place in the inter-war period.