What exactly is white phosphorus and why is it controversial?

Israel has faced accusations this week of using white phosphorus munitions in southern Lebanon, with human rights monitors including Amnesty International and Human Rights Watch documenting extensive Israeli use of the weapon over populated areas in Gaza and Lebanon since October 2023. White phosphorus causes severe burns and suffocation; while not explicitly banned under international law, its use in populated areas violates international humanitarian law principles prohibiting indiscriminate attacks, according to legal experts including Harvard's Bonnie Docherty.