White phosphorus is a chemical substance dispersed in artillery shells, bombs, and rockets that ignites when exposed to oxygen. Its incendiary effects inflict death or cruel injuries that result in lifelong suffering. It can set homes, agricultural areas, and other civilian objects on fire. Under international humanitarian law, the use of airburst white phosphorus is unlawfully indiscriminate in populated areas and otherwise does not meet the legal requirement to take all feasible precautions to avoid civilian harm.
... Lebanon should promptly file a declaration with the International Criminal Court (ICC), enabling the investigation and prosecution of grave international crimes within the court’s jurisdiction on Lebanese territory since October 2023.
There is no per se prohibition on the use of white phosphorous. For instance, a March 2009 HRW report notes that “[w]hen used properly in open areas, white phosphorous munitions are not illegal.” A 2017 article in the New York Times likewise noted that “it is not illegal under international law for militaries to possess and use white phosphorus.” The military manuals of several States indicate that it may be used lawfully, even as an anti-personnel weapon, in certain circumstances (e.g., United States (§ 6.14.2.1), Canada (para. 521.3.), France (p. 20-21), Germany (paras. 453-458), and Australia (paras. 4.30-31)). The question, then, is whether the use of white phosphorous munitions is restricted by weapons treaty law or the law of armed conflict rules governing the conduct of hostilities.
....[E]ven if white phosphorous munitions did qualify as “incendiary weapons,” Protocol III would not ban their use. Rather, it regulates the use of incendiary weapons by parties to the instrument for the purpose of protecting civilians.
The US Army War Manual says "[W]hite phosphorus may be used as an antipersonnel weapon. However, such use must comply with the general rules for the conduct of hostilities, including the principles of discrimination and proportionality.In addition, feasible precautions to reduce the risk of harm to civilians must be taken."
Israel says it only uses shells with WP in urban areas under very specific (undisclosed) circumstances that have been approved by Israel's High Court. While the specific use cases are secret, we could get some clues from the footnotes in the US Army Manual, which says the army used white phosphorus in urban areas in Fallujah directly against terrorists: "We used it for screening missions at two breeches and, later in the fight, as a potent psychological weapon against the insurgents in trench lines and spider holes when we could not get effects on them with HE [High Explosive]. We fired ‘shake and bake’ missions at the insurgents, using WP to flush them out and HE to take them out.”
The Lieber article quotes the Protocol on Prohibitions or Restrictions on the Use of Incendiary Weapons, which says that even as an incendiary weapon, it is permitted "when such military objective is clearly separated from the concentration of civilians and all feasible precautions are taken with a view to limiting the incendiary effects to the military objective and to avoiding, and in any event to minimizing, incidental loss of civilian life, injury to civilians and damage to civilian objects.