After reviewing the “Law of Land Warfare” and more specifically
the U.S.M.C. FM 27-10, MCRP 5-12.1A it appears that the Marines
use of white phosphorous on insurgents and terrorists in
Fallujah was not in violation of U.S. or any international law
that the United States is a signatory to. Of course this does
not make the deaths of the Iraqi insurgents and foreign
terrorists any less graphic or horrifying. In the case of
Fallujah, the Marines felt that the situation warranted the use
of WP in this manner. Their actions as it currently stands under
U.S. and international law were lawful.
As stated in the Geneva Protocol of 1925, White phosphorous does
not fall into the category of chemical weapons prohibited under
its conventions.
*Excerpt: The use in war of smoke and incendiary materials is
not prohibited or restricted by the Geneva Protocol of 1925.
The following are excerpts of the previously stated manual on
U.S. and International laws of land warfare:
36. Weapons Employing Fire
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. They should not, however, be employed in such
a way as to cause
unnecessary suffering to individuals.
34. Employment of Arms Causing Unnecessary Injury
a. Treaty Provision.
It is especially forbidden * * * to employ arms, projectiles, or
material calculated to
cause unnecessary suffering. (HR, art. 23, par. (e).)
b. Interpretation. What weapons cause "unnecessary injury" can
only be determined in
light of the practice of States in refraining from the use of a
given weapon because it is
believed to have that effect. The prohibition certainly does not
extend to the use of
explosives contained in artillery projectiles, mines, rockets,
or hand grenades. Usage has,
however, established the illegality of the use of lances with
barbed heads, irregular shaped
bullets, and projectiles filled with glass, the use of any
substance on bullets that
would tend unnecessarily to inflame a wound inflicted by them,
and the scoring of the
surface or the filing off of the ends of the hard cases of
bullets.
https://www.doctrine.usmc.mil/signpubs/r5121a.pdf