Crusher Bob: There is this. It is also mentioned in The Laws of War on Land (1880), but neither explosive/fulminating projectiles under 400 grams nor the Declaration of St Petersburg are mentioned in any later documents. I can only imagine it has since then either been unanimously struck down or simply ignored, or perhaps it was only agreed upon for a certain time in the first place. Certainly by the 1930s nobody was giving it a second thought.

There's also this (including a bit about banning incendiary weapons except when attacking aircraft or single enemy combatants), but it's only a draft and was never ratified by any nation. Likewise there's this (same text annexed to a convention here) which is only really saying that you can't napalm civilian targets -- you could squirm your way out of a Dresden with all the exceptions, but redoing Trang Bang might not be a good idea.