Though white phosphorus shells are not uncommon in the military, and often used as smokescreens, the high temperature at which it burns, and the toxic chemicals emitted makes them wholly unsuitable for populated areas, and their use in any densely populated area or as an incendiary are widely considered war crimes.
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Here is some info about the use of such weapons:
As is often the case, since the US "is not a party to this Protocol" one can claim their use is not a war crime.
However, I can guarantee that if someone dropped burning white phosphorus on YOUR family you would scream bloody murder.
What did Jesus say about this?
And as ye would that men should do to you, do ye also to them likewise.
Luke 6:31 KJVAmen .