White phosphorus munitions are weapons which 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 include WP and the slang term "Willie Pete" or "Willie Peter" derived from William Peter, the World War II phonetic alphabet for "WP", which is still sometimes used in military jargon. White phosphorus is pyrophoric (self-ignites on 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 vapor. As a result, 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 as part of the ammunition allotment for artillery or mortars. These create smoke screens to mask friendly forces' movement, position, infrared signatures, or shooting positions. They are often described as smoke/marker rounds due to their secondary function of marking points of interest, such as a light mortar being used to designate a target area for artillery spotters.
White phosphorus is believed to have been first used by Fenian (Irish nationalist) arsonists in the 19th century, in the form of a solution in carbon disulfide. When the carbon disulfide evaporated, the phosphorus would burst into flames. This mixture was known as "Fenian fire".
In 1916, during an intense struggle over conscription for the First World War, 12 members of the Industrial Workers of the World, a workers union opposed to conscription, were arrested in Sydney, Australia and convicted for using or plotting to use incendiary materials, including phosphorus. It is believed that eight or nine men in this group, known as the Sydney Twelve, had been framed by the police. Most were released in 1920 after an inquiry.
The British Army introduced the first factory-built WP grenades in late 1916. During World War I, 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 British military also used white phosphorus bombs against Kurdish villagers and Al-Habbaniyah in Al-Anbar province during the Great Iraqi Revolution of 1920.
In the interwar years, the US Army trained using white phosphorus, by artillery shell and air bombardment.
In 1940, when the invasion of the British home islands seemed imminent, the phosphorus firm of Albright and Wilson suggested that the British government use a material similar to Fenian fire in several expedient incendiary weapons. The only one fielded was the Grenade, No. 76 or Special Incendiary Phosphorus grenade, which consisted of a glass bottle filled with a mixture similar to Fenian fire, plus some latex. It came in two versions, one with a red cap intended to be thrown by hand, and a slightly stronger bottle with a green cap, intended to be launched from the Northover projector (a crude 2.5-inch launcher using black powder as a propellant). These were improvised anti-tank weapons, hastily fielded in 1940 when the British were awaiting a German invasion after losing the bulk of their modern armaments in the Dunkirk evacuation.
At the start of the Normandy campaign, 20% of American 81 mm mortar ammunition consisted of M57 point-detonating bursting smoke rounds using WP filler. At least five American Medal of Honor citations mention their recipients using M15 white phosphorus hand grenades to clear enemy positions, and in the 1944 liberation of Cherbourg alone, a single US mortar battalion, the 87th, fired 11,899 white phosphorus rounds into the city. The US Army and Marines used M2 and M328 WP shells in 107mm (4.2 inch) mortars. White phosphorus was widely used by Allied soldiers for breaking up German attacks and creating havoc among enemy troop concentrations during the latter part of the war.
US Sherman tanks carried the M64, a 75mm white phosphorus round intended for screening and artillery spotting, but tank crews found it useful against German tanks such as the Panther that their APC ammunition could not penetrate at long range. Smoke from rounds fired directly at German tanks would be used to blind them, allowing the Shermans to close to a range where their armour-piercing rounds were effective. In addition, due to the turret ventilation systems sucking in fumes, German crews would sometimes be forced to abandon their vehicle: this proved particularly effective against inexperienced crews who, on seeing smoke inside the turret, would assume their tank had caught fire. Smoke was also used for "silhouetting" enemy vehicles, with rounds dropped behind them to produce a better contrast for gunnery.
White phosphorus munitions were used extensively in Korea, Vietnam and later by Russian forces in First Chechen War and Second Chechen War. White phosphorus grenades were used in Vietnam for destroying Viet Cong tunnel complexes as they would burn up all oxygen and suffocate the enemy soldiers sheltering inside. British soldiers also made extensive use of phosphorus grenades during the Falklands conflict to destroy Argentine positions as the peaty soil they were constructed from tended to lessen the impact of fragmentation grenades. According to GlobalSecurity.org, during the December 1994 battle for Grozny in Chechnya, every fourth or fifth Russian artillery or mortar shell fired was a smoke or white phosphorus shell.
In April 2004, during the First Battle of Fallujah, Darrin Mortenson of California's North County Times reported that US forces had used white phosphorus as an incendiary weapon while "never knowing what the targets were or what damage the resulting explosions caused". Embedded with the 2nd Battalion, 1st Marine Regiment, Mortenson described a Marine mortar team using a mixture of white phosphorus and high explosives to shell a cluster of buildings where insurgents had been spotted throughout the week.[dead link] In November 2004, during the Second Battle of Fallujah, Washington Post reporters embedded with Task Force 2-2, Regimental Combat Team 7 stated that they witnessed artillery guns firing WP projectiles. The March/April 2005 issue of an official Army publication called Field Artillery magazine reported that "White phosphorus proved to be an effective and versatile munition and a potent psychological weapon against the insurgents in trench lines and spider holes. ... We fired 'shake and bake' missions at the insurgents using W.P. [white phosphorus] to flush them out and H.E. [high explosives] to take them out".
The US Embassy in Rome denied that US troops had used White Phosphorus as a weapon. In November 2005, the US ambassador to the United Kingdom, Robert Tuttle, wrote to The Independent also denying that the United States used white phosphorus as a weapon in Fallujah. However, on 15 November 2005, US Department of Defence spokesman Lieutenant Colonel Barry Venable confirmed to the BBC that US forces had used white phosphorus as an incendiary weapon there. The documentary Fallujah, The Hidden Massacre, produced by RAI TV, claimed that Iraqi civilians, including women and children, had died of burns caused by white phosphorus during the assault on Fallujah. The claim was denied by Venable who also stated that "When you have enemy forces that are in covered positions that your high explosive artillery rounds are not having an impact on and you wish to get them out of those positions, one technique is to fire a white phosphorus round into the position because the combined effects of the fire and smoke - and in some case the terror brought about by the explosion on the ground - will drive them out of the holes so that you can kill them with high explosives."
On 22 November 2005, the Iraqi government stated it would investigate the use of white phosphorus in the battle of Fallujah. On 30 November 2005, General Peter Pace stated that white phosphorus munitions were a "legitimate tool of the military" used to illuminate targets and create smokescreens, saying "It is not a chemical weapon. It is an incendiary. And it is well within the law of war to use those weapons as they're being used, for marking and for screening". Professor Paul Rodgers from the University of Bradford department of peace studies said that white phosphorus would probably fall into the category of chemical weapons if it was used directly against people. George Monbiot stated that he believed the firing of white phosphorus by US forces directly at the combatants in Fallujah in order to flush them out so they could then be killed was in contravention of the Chemical Weapons Convention and, therefore, a war crime. 
During the 2006 Israel-Lebanon conflict, Israel said that it had used phosphorus shells "against military targets in open ground" in south Lebanon. Israel said that its use of these munitions was permitted under international conventions. However, President of Lebanon Émile Lahoud said that phosphorus shells were used against civilians. The first Lebanese official complaint about the use of phosphorus came from Information Minister Ghazi Aridi.
In its early statements regarding the Gaza War of 2008-2009, the Israeli military denied using WP entirely, saying "The IDF acts only in accordance with what is permitted by international law and does not use white phosphorus." Numerous reports from human rights groups during the war indicated that WP shells were being used by Israeli forces in populated areas.
On 5 January the Times reported that telltale smoke associated with white phosphorus had been seen in the vicinity of Israeli shelling. On 12 January it was reported that more than 50 patients in Nasser Hospital were being treated for phosphorus burns. 
On 15 January, the headquarters of the United Nations Relief and Works Agency in Gaza City was struck by submunitions from Israeli artillery shells, setting fire to pallets of relief materials and igniting several large fuel storage tanks. A UN spokesperson indicated that there were difficulties in extinguishing the fires, stating "You can't put it [white phosphorus] out with traditional methods such as fire extinguishers. You need sand but we do not have any sand in the compound." Senior Israeli defense officials maintain that the shelling was in response to Israeli military personnel being fired upon by Hamas fighters who were in proximity to the UN headquarters, and was used for smoke. The soldiers who ordered the attack were later reprimanded for violating the IDF rules of engagement. The IDF further investigated improper use of WP in the conflict, particularly in one incident in which 20 WP shells were fired in a built-up area of Beit Lahiya.
The Israeli government released a report in July 2009 that confirmed that the IDF used white phosphorus in both exploding munitions and smoke projectiles. The report argues that the use of these munitions was limited to unpopulated areas for marking and signalling and not as an anti-personnel weapon. The Israeli government report further stated that smoke screening projectiles were the majority of the munitions containing white phosphorus employed by the IDF and that these were very effective in that role. The report states that at no time did IDF forces have the objective of inflicting any harm on the civilian population.
Head of the UN Fact Finding Mission Justice Richard Goldstone presented the report of the Mission to the Human Rights Council in Geneva on 29 September 2009. The Goldstone report accepted that white phosphorus is not illegal under international law but did find that the Israelis were "systematically reckless in determining its use in built-up areas". It also called for serious consideration to be given to the banning of its use in built-up areas. The Government of Israel issued an initial response rejecting the findings of the Goldstone report.
The 155mm WP artillery shells used by Israel are typically the American M825A1, a base-ejection shell which deploys an airbursting submunition canister. On detonation of the bursting charge, the canister deploys 116 three-quarter inch, quarter-circle wedges of felt impregnated with 12.75 pounds of WP, producing a smokescreen lasting 5-10 minutes depending on weather conditions. These submunitions typically land in an elliptical pattern 125-250 meters in diameter, with the size of the effect area depending on the burst height, and produce a smokescreen 10 metres in height.
There are confirmed cases of white phosphorus burns on bodies of civilians wounded during US-Taliban clashes near Bagram. The United States has accused Taliban militants of using white phosphorus weapons illegally on at least 44 occasions. In May 2009, Colonel Gregory Julian, a spokesman for General David McKiernan, the overall commander of US and NATO forces in Afghanistan, confirmed that Western military forces in Afghanistan use white phosphorus in order to illuminate targets or as an incendiary to destroy bunkers and enemy equipment. The Afghan government later launched an investigation into the use of white phosphorus munitions.
After the 2016 Armenian-Azerbaijani clashes over disputed territory of Nagorno-Karabakh Azerbaijani Ministry of Foreign Affairs stated that on 10 May of that year the Armenian military had fired 122mm white phosphorus artillery munitions against Azerbaijani territory.
The Syrian government, the United States the Russian Federation and Turkey reportedly deployed white phosphorus munitions via airstrikes and artillery on different occasions during the Syrian Civil War.
Weight-for-weight, phosphorus is the most effective smoke-screening agent known, for two reasons: first, it absorbs most of the screening mass from the surrounding atmosphere and secondly, the smoke particles are actually an aerosol, a mist of liquid droplets which are close to the ideal range of sizes for Mie scattering of visible light. This effect has been likened to three dimensional textured privacy glass—the smoke cloud does not obstruct an image, but thoroughly scrambles it. It also absorbs infrared radiation, allowing it to defeat thermal imaging systems.
When phosphorus burns in air, it first forms phosphorus pentoxide (which exists as tetraphosphorus decoxide except at very high temperatures):
Since an atom of phosphorus has an atomic mass of 31 but a molecule of phosphoric acid has a molecular mass of 98, the cloud is already 68% by mass derived from the atmosphere (i.e. 3.2 kilograms of smoke for every kilogram of WP); however, it may absorb more because phosphoric acid and its variants are hygroscopic. Given time, the droplets will continue to absorb more water, growing larger and more dilute until they reach equilibrium with the local water vapour pressure. In practice, the droplets quickly reach a range of sizes suitable for scattering visible light and then start to dissipate from wind or convection.
Because of the great weight efficiency of WP smoke, it is particularly suited for applications where weight is highly restricted, such as hand grenades and mortar bombs. An additional advantage for hand smoke grenades—which are more likely to be used in an emergency—is that the WP smoke clouds form in a fraction of a second. Because WP is also pyrophoric, most munitions of this type have a simple burster charge to split open the casing and spray fragments of WP through the air, where they ignite spontaneously and leave a trail of rapidly thickening smoke behind each particle. The appearance of this cloud forming is easily recognised; one sees a shower of burning particles spraying outward, followed closely by distinctive streamers of white smoke, which rapidly coalesce into a fluffy, very pure white cloud (unless illuminated by a coloured light source).
Various disadvantages of WP are discussed below, but one which is particular to smoke-screening is "pillaring". Because the WP smoke is formed from fairly hot combustion, the gasses in the cloud are hot, and tend to rise. Consequently the smoke screen tends to rise off the ground relatively quickly and form aerial "pillars" of smoke which are of little use for screening. Tactically this may be counteracted by using WP to get a screen quickly, but then following up with emission type screening agents for a more persistent screen. Some countries have begun using red phosphorus instead. Red phosphorus ("RP") burns cooler than WP and eliminates a few other disadvantages as well, but offers exactly the same weight efficiency. Other approaches include WP soaked felt pads (which also burn more slowly, and pose a reduced risk of incendiarism) and PWP, or plasticised white phosphorus.
In addition to direct injuries caused by fragments of their casings, white phosphorus munitions can cause injuries in two main ways: burn injuries and vapor inhalation.
Incandescent particles from weapons using powdered white phosphorus as their payload produce extensive partial- and full-thickness burns, as will any attempt to handle burning submunitions without protective equipment. Phosphorus burns carry an increased risk of mortality due to the absorption of phosphorus into the body through the burned area with prolonged contact, which can result in liver, heart and kidney damage, and in some cases multiple organ failure. White phosphorus particles continue to burn until completely consumed unless deprived of oxygen. In the case of weapons using felt-impregnated submunitions, incomplete combustion may occur resulting in up to 15% of the WP content remaining unburned. Such submunitions can prove hazardous as they are capable of spontaneous re-ignition if crushed by personnel or vehicles. In some cases, injury is limited to areas of exposed skin because the smaller WP particles do not burn completely through personal clothing before being consumed.
Due to the pyrophoric nature of WP, penetrating injuries are immediately treated by smothering the wound using water, damp cloth or mud, isolating it from oxygen until fragments can be removed: military forces will typically do so using a bayonet or knife where able. Bicarbonate solution is applied to the wound to neutralise any build-up of phosphoric acid, followed by removal of any remaining visible fragments: these are easily observed as they are luminescent in dark surroundings. Surgical debridement around the wound is used to avoid fragments too small to detect causing later systemic failure, with further treatment proceeding as with a thermal burn.
Burning white phosphorus produces a hot, dense, white smoke consisting mostly of phosphorus pentoxide in aerosol form. Field concentrations are usually harmless, but at high concentrations the smoke can cause temporary irritation to the eyes, mucous membranes of the nose, and respiratory tract. The smoke is more dangerous in enclosed spaces, where it can cause asphyxiation and permanent respiratory damage. The US Agency for Toxic Substances and Disease Registry has set an acute inhalation Minimum Risk Level (MRL) for white phosphorus smoke of 0.02 mg/m3, the same as fuel-oil fumes. By contrast, the chemical weapon mustard gas is 30 times more potent: 0.0007 mg/m3. The agency cautioned that studies used to determine the MRL were based on extrapolations from animal testing and may not accurately reflect the health risk to humans. There are no documented instances of fatalities from smoke inhalation alone under combat conditions.
While in general white phosphorus is an industrial chemical not subject to restriction, certain uses in weaponry are banned or restricted by general international laws: in particular, those related to incendiary devices.
Article 1 of Protocol III of the Convention on Certain Conventional Weapons defines an incendiary weapon as "any weapon or munition which is primarily designed to set fire to objects or to cause burn injury to persons through the action of flame, heat, or combination thereof, produced by a chemical reaction of a substance delivered on the target". Article 2 of the same protocol prohibits the deliberate use of incendiary weapons against civilian targets (already forbidden by the Geneva Conventions), the use of air-delivered incendiary weapons against military targets in civilian areas, and the general use of other types of incendiary weapons against military targets located within "concentrations of civilians" without taking all possible means to minimize casualties.
The convention also exempts certain categories of munitions from its definition of incendiary weapons: specifically, these are munitions which "may have incidental incendiary effects, such as illuminants, tracers, smoke or signalling systems" and those "designed to combine penetration, blast or fragmentation effects with an additional incendiary effect."
The use of incendiary and other flame weapons against matériel, including enemy military personnel, is not directly forbidden by any treaty. The United States Military mandates that incendiary weapons, where deployed, not be used "in such a way as to cause unnecessary suffering." The term "unnecessary suffering" is defined through use of a proportionality test, comparing the anticipated military advantage of the weapon's use to the amount of suffering potentially caused.
The Chemical Weapons Convention, sometimes invoked in discussions of WP usage, is meant to prohibit weapons that are "dependent on the use of the toxic properties of chemicals as a method of warfare" (Article II, Definitions, 9, "Purposes not Prohibited" c.). The convention defines a "toxic chemical" as a substance "which through its chemical action on life processes can cause death, temporary incapacitation or permanent harm to humans or animals" (CWC, II). An annex lists chemicals that are restricted under the convention, and WP is not listed in the Schedules of chemical weapons or precursors.
In a 2005 interview with RAI, Peter Kaiser, spokesman for the Organisation for the Prohibition of Chemical Weapons (an organization overseeing the CWC and reporting directly to the UN General Assembly), discussed cases where use of WP would potentially fall under the auspices of the CWC:
No it's not forbidden by the CWC if it is used within the context of a military application which does not require or does not intend to use the toxic properties of white phosphorus. White phosphorus is normally used to produce smoke, to camouflage movement.
If that is the purpose for which the white phosphorus is used, then that is considered under the convention legitimate use.
If on the other hand the toxic properties of white phosphorus are specifically intended to be used as a weapon, that of course is prohibited, because the way the convention is structured or applied, any chemicals used against humans or animals that cause harm or death through the toxic properties of the chemical are considered chemical weapons.