Human Rights Watch accused the Israeli military of "deliberately or recklessly" using white phosphorus shells over densely populated civilian areas, in a report released Wednesday. The Israel Defense Forces are not alone in their use of white phosphorus—during the 2004 battle of Fallujah, for example, U.S. troops used the incendiary weapon. If white phosphorus gets human rights organizations so upset, why do militaries keep using it?
Because it's versatile. White phosphorus, known in martial circles as "Willy Pete," may cause horrific burns when used as a weapon (click here for a gruesome visual and here [PDF] for a report on its health effects), but it's primarily deployed as a smoke screen. When packed into munitions, the chemical serves as a particularly efficient masking agent because it works so quickly—bursting into thick, white, billowing clouds in just a fraction of a second. The M116 shell, by contrast, which is also used in military campaigns, takes approximately two minutes to produce smoke. White phosphorus also impedes the use of infrared tracking systems, like those used to guide some anti-tank missiles. Finally, it's light in weight, which makes it suitable as a filling for hand grenades.
White phosphorus has an iffy legal status. No treaty bans the use of white phosphorus against strictly military targets, although there is a debate over whether it ought to be classified as a chemical weapon and prohibited under the Chemical Weapons Convention. Protocol III of the Convention on Certain Conventional Weapons outlaws the use of "incendiary weapons" on civilians or on military targets located within a concentration of civilians. The protocol, however, specifically excludes munitions that "may have incidental incendiary effects, such as illuminants, tracers, smoke or signaling systems." Moreover, Israel is not a Protocol III signatory; and the United States didn't sign until January of this year. *
Bonus Explainer: Does phosphorus come in colors other than white? Yes, there are three allotropic forms of phosphorus: white (or yellow), red, and black. White phosphorus, which smells kind of like garlic, is a thermodynamically unstable, waxy solid that starts to yellow when exposed to heat or sunlight. If exposed to oxygen, it ignites spontaneously. Red phosphorus is more stable, catching fire only above 240 degrees Celsius, and can be used in smoke bombs, but it won't double as an incendiary. (For this reason, some countries have begun using it as an alternative to white phosphorus.) Black phosphorus looks kind of like graphite—dark and flaky—and is the least reactive.
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Explainer thanks Ronald Breslow of Columbia University and Marc Garlasco of Human Rights Watch.