Why does this matter?
The use of white phosphorus is a war crime under international treaties and domestic U.S. laws. For example, the Battle Book, published by the U.S. Command and General Staff College at Fort Leavenworth, Kansas, contains the following sentence:
It is against the law of land warfare to employ WP [i.e. white phosphorous] against personnel targets.
And the U.S. called white phosphorous a chemical weapon when Saddam used it against the Kurds.
Unfortunately, this is not the first time … The U.S. has repeatedly used illegal chemical weapons.