The Nebelwerfer (smoke mortar) was a World War II German series of weapons. They were initially developed by and assigned to the Wehrmacht's "smoke troops" (Nebeltruppen). This weapon was given its name as a disinformation strategy designed to fool observers from the League of Nations, who were observing any possible infraction of the Treaty of Versailles, into thinking that it was merely a device for creating a smoke screen.They were primarily intended to deliver poison gas and smoke shells, although a high-explosive shell was developed for the Nebelwerfer from the beginning. Initially, two different mortars were fielded before they were replaced by a variety of rocket launchers ranging in size from 15 to 32 centimetres (5.9 to 12.6 in). The thin walls of the rockets had the great advantage of allowing much larger quantities of gases, fluids or high explosives to be delivered than artillery or even mortar shells of the same weight. With the exception of the Balkans Campaign, Nebelwerfer were used in every campaign of the German Army during World War II. A version of the 21 cm calibre system was adapted for air-to-air use against Allied bombers.