As an incendiary weapon, WP burns fiercely and can set cloth, fuel, ammunition and other combustibles on fire. Since WWII, it has been extensively used as an anti-personnel weapon capable of causing serious burns or death. The agent is used in bombs, artillery, and mortars, short-range missiles which burst into burning flakes of phosphorus upon impact. White phosphorus is commonly referred to in military jargon as "WP", and the slang term "Willy/Willie Pete/Peter" (dating from World War I and common at least through the Vietnam War) is still sometimes used by infantry and artillery servicemen.
WP is also a highly efficient smoke producing agent, burning quickly and causing an instant bank of smoke. As a result, smoke producing WP munitions are very common, particularly as smoke grenades for infantry, loaded in defensive grenade dischargers on tanks and other armored vehicles, or as part of the ammunition allotment for artillery or mortars. These create smokescreens to mask movement from the enemy, or to mask his fire.
WP is believed to have been first used by Fenian arsonists in the 19th century in the form of a solution of WP in carbon disulfide. When the carbon disulfide evaporated, the WP would burst into flames, and probably also ignite the highly flammable carbon disulfide fumes. This mixture was known as "Fenian fire" and allegedly was used by disgruntled itinerant workers in Australia to cause delayed destruction of shabby sleeping quarters.
In 1916, during an intense ideological struggle over conscription for the First World War, twelve members of the I.W.W., a radical union of workers who openly opposed conscription, were arrested 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. In World War II, 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. In 1940, when the invasion of Britain 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 (cf. Molotov cocktail, Greek fire). 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 blackpowder grenade launcher). 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 France in May 1940. Instructions on each crate of SIP grenades included the observations, inter alia:
It was generally regarded as overly dangerous to its own operators and was never deployed in combat.
At the start of the Normandy campaign, 20% of American 81 mm mortar rounds were WP. At least five American Medal of Honor citations mention their recipients using white phosphorus grenades to clear enemy positions. In the 1944 liberation of Cherbourg alone, a single U.S. mortar battalion, the 87th, fired 11,899 white phosphorus rounds into the city.
The U.S. Army and Marines used WP shells in 107-mm [4.2 Inch] mortars. WP was widely credited by Allied soldiers for breaking up German infantry attacks and creating havoc among enemy troop concentrations during the latter part of the war. American servicemen in the Pacific and otherwise (to this day) were known to call the thrown bottles "Willie Pete" grenades. The origin of the term has been thought to be derived from the British military's phonetic alphabet.
Incendiary bombs were used extensively by the German, British and US air forces against civilian populations and targets of military significance in civilian areas (London, Hamburg, Dresden, Area bombing etc). Late in the war, some of these bombs used white phosphorus (about 1-200 grams) in place of magnesium as the igniter for their flammable mixtures. The use of incendiary weapons against civilians was banned (by signatory countries) in the 1980 Convention on Certain Conventional Weapons Protocol III. The USA signed Protocols I and II on March 24, 1995 (and the amended article II on May 24, 1999) and later Protocols III, IV, and V, on January 21, 2009.
WP munitions were used extensively in Korea, Vietnam and later by Russian forces in Chechnya. According to GlobalSecurity.org, during the December 1994 battle for Grozny in Chechnya, every fourth or fifth Russian artillery or mortar round fired was a smoke or white phosphorus round. 
In Iraq, the Saddam Hussein regime used white phosphorus, as well as chemical weapons that are scheduled in the Chemical Weapons Convention, in the Halabja poison gas attack during the Iran–Iraq War in 1988, according to the ANSA news agency.
Another news report  said "US intelligence" called WP a chemical weapon in a declassified Pentagon report from February 1991:
but the actual declassified document contains the words "WARNING: (U) THIS IS AN INFORMATION REPORT, NOT FINALLY EVALUATED INTELLIGENCE." By "information report", the document states it is not a reviewed product of the intelligence community. Further, the document's addressee codes all start with the letter R, which means that they are in the military operational community, not the Y-community that is reserved for the intelligence community (Chapter IV, Section 11).
Use of WP against enemy areas in Fallujah were reported as early as April 2004:
This U.S. Department of State website carried an addendum in November 2005, replacing the previous statement with the comment:
The specific aspect of use against humans was highlighted after the documentary film Fallujah, The Hidden Massacre by Sigfrido Ranucci was aired on Italy's RaiNews24 and released on the internet. In the film, Giuliana Sgrena quotes city refugees testimonies from Fallujah about the reported danger of weapons effects:
The film also shows U.S. soldiers on film confirming to WP use against insurgents. U.S. ambassador to UK Robert Holmes Tuttle stated in November 2005, that U.S. forces "do not use napalm or white phosphorus as weapons".
However, within a week of ambassador Tuttle's statement, on November 15, Pentagon spokesman Lieutenant-Colonel Barry Venable confirmed to the BBC that WP had been used as an antipersonnel weapon, and was quoted as stating: "It has been used as an incendiary weapon against enemy combatants". In particular,
WP use is legal for purposes such as illumination and obscuring smoke, and the Chemical Weapons Convention does not list WP in its schedules of chemical weapons.
The March 2005 edition of the U.S. Army magazine Field Artillery, contained an article on using white phosphorus as an "effective munition" for flushing out insurgents during the Fallujah attack of November 2004:
On November 30, 2005, General Peter Pace defended use of WP, declaring that WP munitions were a "legitimate tool of the military", used to illuminate targets and create smokescreens, and that there were better weapons for killing people:
On June 22, 2007 New York Times correspondent Michael R. Gordon was interviewed on National Public Radio in a story called "Baquba Residents Displaced by Insurgents" by Melissa Block and Michele Norris. In this interview, Gordon was asked about civilian casualties in Baquba, Iraq. He responded by saying "Yeah, there have been civilian casualties. I was just talking to our photographer and he had seen people who are hurt by phosphorus shells." The photographer was not identified in the interview and the report was not corroborated.
During the 2006 Israel-Lebanon conflict, Israel stated that it had used phosphorus shells "against military targets in open ground" in south Lebanon. Israel stated that its use of the white phosphorus bombs was permitted under international conventions. President of Lebanon Émile Lahoud claimed that phosphorus shells were used against civilians in Lebanon. The first Lebanese official complaint about the use of phosphorus came from Information Minister Ghazi Aridi.
Several reports from human right groups during the war indicated that white phosphorus shells were being used by Israel in violation of international law. Human Rights Watch said shells exploded over populated civilian areas, including a crowded refugee camp and a United Nations school where civilians were seeking refuge. Additionally, Human Rights Watch said that white phosphorus injuries were suspected in the cases of ten burn victims.. Additionally, The International Red Cross said that Israel has fired white phosphorus shells in its offensive in the Gaza Strip, but has no evidence to suggest it is being used improperly or illegally
Human Rights Watch said its experts in the region had witnessed the use of white phosphorus. Kenneth Roth, the organisation's executive director, added: "This is a chemical compound that burns structures and burns people. It should not be used in populated areas."
Amnesty International said a fact-finding team found "indisputable evidence of the widespread use of white phosphorus" in crowded residential areas of Gaza City and elsewhere in the territory. Donatella Rovera, the head of an Amnesty fact-finding mission to southern Israel and Gaza, said: "Israeli forces used white phosphorus and other weapons supplied by the USA to carry out serious violations of international humanitarian law, including war crimes."
On January 14, the Israelli newspaper Haaretz claimed that Hamas had fired a white phosphorus mortar shell which exploded in an open area in the Eshkol area in the western Negev of southern Israel. No injuries or damage were reported by this newspaper.
On 15 January, the United Nations compound, housing numerous refugees in Gaza City, was struck by Israeli white phosphorus 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 attempting to extinguish the fires because of the white phosphorus and stated "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. The Israeli army purports to be investigating improper use of WP in this conflict, particularly in one incident in which 20 WP shells were fired in a built-up area of Beit Lahiya.
On 17 January, Peter Herby, head of the International Committee of the Red Cross Arms Unit , confirmed the use of white phosphorus weapons by Israel in Gaza, outlined the rules applicable to phosphorus weapons and explained the ICRC's approach to the issue.
On January 20, Paul Wood of the BBC reports from Gaza on what appears to be white phosphorus use near the vicinity of civilian areas. Amnesty team weapon expert Christopher Cobb-Smith, who witnessed the shelling during the conflict, reported "we saw streets and alleyways littered with evidence of the use of white phosphorus, including still-burning wedges and the remnants of the shells and canisters fired by the Israeli army." 
On March 25, 2009, USA Based Human Rights Organization Human Rights Watch published a 71 page report titled Rain of Fire, Israel’s Unlawful Use of White Phosphorus in Gaza and claimed the Israel's usage of the weapon was illegal.
This 71-page report provides witness accounts of the devastating effects that white phosphorus munitions had on civilian targets and civilian property in Gaza. Human Rights Watch researchers in Gaza immediately after hostilities ended found spent shells, canister liners, and dozens of burnt felt wedges containing white phosphorus on city streets, apartment roofs, residential courtyards, and at a United Nations school. The report also presents ballistics evidence, photographs, and satellite imagery, as well as documents from the Israeli military and government. 
The Israeli government released a report in July 2009 that admitted that the IDF used white phosphorus in exploding munitions and smoke projectiles. The report stated that the use of exploding munitions were used by Israeli ground and naval forces. The report defended the use of these munition claiming that they were only fired on unpopulated areas for marking and signaling and not as an anti-personnel weapon despite contrary eyewitness testimony and physical evidence that these weapons had been used against civilians. The report further says that the main type of munitions containing white phosphorus employed by the IDF during the Gaza Operation was smoke screening projectiles, which are smoke shells containing felt wedges dipped in white phosphorus. The report suggests that the use of smoke obscurants proved to be highly effective at cloaking IDF forces while obstructing enemy lines of sight and claims that at no time did IDF forces have the objective of inflicting any harm on the civilian population.
An article by Mark Cantora, published in 2010 in the Gonzaga Journal of International Law, argues that Israel's use of White Phosphorus in Gaza was legal under relevant international humanitarian laws, and that the international community's failure to enact less ambiguous laws banning white phosphorus is to blame for any civilian injuries suffered as a result of Israel's white phosphorus use.
There are confirmed cases of white phosphorus burns on bodies of civilians wounded in Afganistan 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 U.S. 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.
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 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 simply obstruct an image, but thoroughly scrambles both visual and infrared radiation, interfering with infra-red optics and weapon-tracking systems, serving as a protection for military forces from guided weapons such as anti-tank missiles.
When phosphorus burns in air, it first forms diphosphorus 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 you started with); 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.
White phosphorus can cause injuries and death in three ways: by burning deep into tissue, by being inhaled as a smoke, and by being ingested. Extensive exposure by burning and ingestion is fatal.
Incandescent particles of WP cast off by a WP weapon's initial explosion can produce extensive, deep second and third degree burns. One reason why this occurs is the tendency of the element to stick to the skin. Phosphorus burns carry a greater risk of mortality than other forms of burns due to the absorption of phosphorus into the body through the burned area, resulting in liver, heart and kidney damage, and in some cases multiple organ failure. These weapons are particularly dangerous to exposed people because white phosphorus continues to burn unless deprived of oxygen or until it is completely consumed. In some cases, burns are limited to areas of exposed skin because the smaller WP particles do not burn completely through personal clothing before being consumed.
Burning WP produces a hot, dense, white smoke consisting mostly of phosphorus pentoxide. Most forms of the smoke are not hazardous in the likely concentrations produced by a battlefield smoke shell. Exposure to heavy smoke concentrations of any kind for an extended period (particularly if near the source of emission) does have the potential to cause illness or even death.
WP smoke irritates the eyes, mucous membranes of the nose, and respiratory tract in moderate concentrations, while higher concentrations may produce severe burns. However, no casualties have been recorded from the effects of WP smoke alone in combat operations and there are no confirmed deaths resulting from exposure to phosphorus smoke.
The 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 accepted lethal dose when white phosphorus is ingested orally is 1 mg per kg of body weight, although the ingestion of as little as 15 mg has resulted in death. It may also cause liver, heart or kidney damage. There are reports of individuals with a history of oral ingestion who have passed phosphorus-laden stool ("smoking stool syndrome")
There are multiple international laws that could be seen regulate WP use. 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'. The same protocol also prohibits the use of incendiary weapons against civilians (already forbidden by the Geneva Conventions) or in civilian areas.
However, the use against military targets outside civilian areas is not explicitly banned by any treaty. There is a debate on whether white phosphorus should be considered a chemical weapon and thus be outlawed by the Chemical Weapons Convention (CWC) which went into effect in April 1997. The convention 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 chemical "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 fall under this definition and WP is not listed in the Schedules of chemical weapons or precursors.
In an 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), questioned whether the weapon should fall under the convention's provisions:
Kaiser was a staff spokesman for the Organisation for the Prohibition of Chemical Weapons. The OPCW, using member votes, creates Schedules of chemical weapons or dual-use chemicals of concern and white phosphorus is not in any of these schedules.
The Convention on Certain Conventional Weapons, not the Chemical Weapons Convention, goes on, in its Protocol III, to prohibit the use of all air-delivered incendiary weapons against civilian populations, or for indiscriminate incendiary attacks against military forces co-located with civilians. However, that protocol also specifically excludes weapons whose incendiary effects are secondary, such as smoke grenades. This has often been read as excluding white phosphorus munitions from this protocol, as well. Several countries, most notably Israel, are not signatories to Protocol III.
The legal position however, is not the only consideration in any war. For instance, concerning the U.S. use of WP in Iraq, the British Liberal Democrat foreign affairs spokesman Sir Menzies Campbell, said
Within the US Army, there appears to be conflicting advice on the use of WP against humans. According to the field manual on the Rule of Land Warfare, "The use of weapons which employ fire, such as tracer ammunition, flamethrowers, napalm and other incendiary agents, against targets requiring their use is not violative of international law." However, the ST 100-3 Battle Book, a student text published by the US Command and General Staff College at Fort Leavenworth states that "It is against the law of land warfare to employ WP against personnel targets." At the same time, other field manuals discuss the use of white phosphorus against personnel.
Though white phosphorus is still used in modern armed conflict, it is regulated by international humanitarian law, or the law of war.
White phosphorus is an incendiary weapon. It makes a bright light and smoke. Its main ingredient is one of the allotropes of the chemical element phosphorus. White phosphorus is toxic to humans. It can burn deep into soft tissue, it can be breathed in, or it can be ingested (eaten). Over time, these can cause death.
The use of incendiary weapons against civilians is forbidden since 1949. Were used by U.S. forces in Iraq in 2004. White Phosphorous was also used in the Israeli invasion of Gaza during December 2008 through January 2009. These actions were supported by the US government.