The Bermgmann MP 18 was a submachine gun introduced to the German army in 1918 and has been called "the first effective submachine gun". A lot of sources tell me that this weapon was banned by the Treaty of Versailles. I have looked through the text of the Treaty but cannot find the relevant passages. The Treaty does talk about "light machine guns", but provides no definition of what this term means, or any other terms for that matter.
According to Paul Cornish's book Machine Guns and the Great War, this is almost certainly untrue. The MP-18 was not produced in significant numbers before the Armistice, perhaps 2-5000 at best, and would not have attracted such attention. Light machine guns like refers to light automatic weapons such as the Lewis Gun or the Chauchat, which would have been familiar to the Allied commission. The MG-08 and MG-08/15 used by the German army were heavy and medium machine guns, the MG-08/15 weighing about as much as the British Vickers Gun, despite it's being meant as a light machine gun for the German Army in WWI.