It turns out: governments lie.

Shocking, like when I realized water is wet.

Who would have thought they'd lie to us - shocking! /s

These "forbidden" substances are usually too hard for a medic to treat and kill slowly. In this case, the thing stick and can burn till the bones, and even after the initial impact the remnants can still ignite. It is just a mess in the body.

Upon contact, white phosphorus can burn people, thermally and chemically, down to the bone as it is highly soluble in fat and therefore in human flesh. White phosphorus fragments can exacerbate wounds even after treatment and can enter the bloodstream and cause multiple organ failure. Already dressed wounds can reignite when dressings are removed and the wounds are re-exposed to oxygen. Even relatively minor burns are often fatal. For survivors, extensive scarring tightens muscle tissue and creates physical disabilities. The trauma of the attack, the painful treatment that follows, and appearance-changing scars lead to psychological harm and social exclusion.

Just to reiterate, cuz it really sounds like some sci-fi alien shit:

Already dressed wounds can reignite when dressings are removed and the wounds are re-exposed to oxygen.

Think Napalm's meaner acidic cousin

What makes this worse than other things?

I think it's good to read the article, they explain a lot of it and how Israel is refusing to comply with its ban as a weapon.

Sorry, you were right, it doesn't explain too much in the article, but this from HRW explains it in much better details: https://www.hrw.org/news/2023/10/12/questions-and-answers-israels-use-white-phosphorus-gaza-and-lebanon

I caused confusion, it was my bad.

Chemical rounds meant to cause injury are banned in the Geneva convention. It's not a separate treaty.

If it's being used as a smokescreen, a very common use, there is nothing at all bad about it.

Edit: People seem to think that it's common use as a smoke screen is a problem. What, specifically, is the issue when it is being used as a smokescreen?

WaPo confirmed it. And an air bursting Magnesium round is also a war crime.

WP burns yellower, and magnesium is very rare because it's expensive and complicated. WP auto-ignites in air, magnesium has to be separately lit by the delivery system. You can't practically put a magnesium incendiary in an artillery shell, what you see in Gaza is mostly artillery.

And suddenly switching to magnesium would be an absolute psycho move. The way you treat a WP burn is pouring tons of water on it, magnesium explodes on contact with water.

WP is not forbidden by international law, but only close to civilians

But isn't basically everywhere in Gaza close to civilians? Gaza city has about the same population density as Boston.

Pro tip: you can't head off claims of whataboutism my preemptively trivializing them. It doesn't work that way.

WP is forbidden as an incendiary weapon, not as a smoke screen or for illumination. I know the distinction seems silly, but there are actually no WP incendiary weapons in NATO inventories anymore. I don't know about Israel, but I assume the same is true since they have a lot of the same supply chains.

There is a pretty big functional difference though. WP incendiary rounds are basically cluster munitions intended to contain a solid chunk of relatively slow burning WP so that it can settle on objects and remain in contact for an extended period while it burns. This is why it's such a horrific weapon - because it will literally burn through roofs and people and keep burning.

WP smoke and illumination rounds can obviously light fires as well, but they are intended to burn much quicker to produce greater volumes of smoke and light. While you definitely don't want to get a coating of burning WP powder on your roof, it's much less dangerous than a golfball sized sub muntion which will spend a solid 10 minutes at 3000F.

None of the alleged videos of Israel using WP show incendiary rounds IMO. They are all illumination rounds. Though they do often seem to be deployed in a way which is more likely to make fires. Russia does the same thing. They use illumination rounds in a direct fire capacity to blind and discombobulate. This is still probably a war crime.

WP with a design meant to create shrapnel is absolutely banned. The only legal Phosphorus rounds are ones that have a small bursting charge after striking the ground.