It’s about trying to prevent the worst and most horrific actions and outcomes that happen during war.

No. It’s about trying to prevent militarily unnecessary worst and most horrific actions and outcomes.

White Phosphorus as incendiary rounds,

Perfectly legal. You can’t use them as chemical rounds (they’re shit at that anyways), or, as any other incendiary weapons, close to civilians. By far the most common use is as tracer rounds and in smokescreens, though.