Who makes up these rules of war?

So, we keep hearing about Israel and its possible use of chemical weapons or that it broke the international rules of war in Gaza by using a substance called white phosphorus.

Of course, Israel denies that it broke international war rules (is there a book or something?). Some sources say that only a little bit was used and not in the amounts that are illegal (am I missing something here? Isn't all killing illegal?). Others point to evidence of foul play because of the type of medical treatment needed -- "We have seen people who suffered from burns that can only be caused by white phosphorus," said a Jordanian paediatrician who was in Gaza last week.

Anyway, I feel like I am missing something with these allegations. What are these rules of war? War is war, no? Who decides on the rules of war and why do countries have to follow them? Surely any act of war is illegal?

I would just love to get my hands on the "Idiots Guide to Warmongering." Who wrote these rules? Europeans? Maybe some Arabs or Israelis?

I just have this image of boring bureaucrats or ex-army personnel getting together to discuss what is fair play in war and what is not.
"You can shoot a person in the head, that is fine," says one. "But chemicals ... now that is really, really bad."

Now, I do not profess to be an expert on military matters but it does strike me that perhaps if the whole world banned guns, rockets and anything else that kills a person as well as the chemicals then perhaps war would end?

Its just a suggestion and maybe I am just missing point?