White phosphorus munitions are weapons that use one of the common allotropes of the chemical element phosphorus. White phosphorus is used in smoke, illumination, and incendiary munitions, and is commonly the burning element of tracer ammunition. Other common names for white phosphorus munitions include WP and the slang terms Willie Pete and Willie Peter, which are derived from William Peter, the World War II phonetic alphabet rendering of the letters WP. White phosphorus is pyrophoric (it is ignited by contact with air); burns fiercely; and can ignite cloth, fuel, ammunition, and other combustibles. In addition to its offensive capabilities, white phosphorus is a highly efficient smoke-producing agent, reacting with air to produce an immediate blanket of phosphorus pentoxide vapour. Smoke-producing white phosphorus munitions are very common, particularly as smoke grenades for infantry, loaded in defensive grenade launchers on tanks and other armoured vehicles, and in the ammunition allotment for artillery and mortars. These create smoke screens to mask friendly forces' movement, position, infrared signatures, and shooting positions. They are often called smoke/marker rounds for their use in marking points of interest, such as a light mortar to designate a target for artillery spotters. Early use White phosphorus was used by Fenian (Irish nationalist) arsonists in the 19th century in a formulation that became known as "Fenian fire". The phosphorus would be in a solution of carbon disulfide; when the carbon disulfide evaporates, the phosphorus bursts into flames. The same formula was also used in arson in Australia. World War I, the inter-war period and World War II The British Army introduced the first factory-built white phosphorus grenades in late 1916 during the First World War. During the war, white phosphorus mortar bombs, shells, rockets, and grenades were used extensively by American, Commonwealth, and, to a lesser extent, Japanese forces, in both smoke-generating and antipersonnel roles. The Royal Air Force based in Iraq also used white phosphorus bombs in Anbar Province during the Iraqi revolt of 1920. Among the many social groups protesting the war and conscription at the time, at least one, the Industrial Workers of the World in Australia, used Fenian fire. In the interwar years, the US Army trained using white phosphorus, by artillery shell and air bombardment. In 1940, when the German invasion of Great Britain seemed imminent, the phosphorus firm of Albright and Wilson suggested that the British government use a material similar to Fenian fire