Fairly Philosophical Friday: Where can I get the rule book for war?

Yesterday the President gave a speech defending his use of drone strikes in the theater of war. He gave that speech in part to answer his critics who think such drone strikes are unethical/illegal. Which is curious. Apparently, there are rules for the manner in which you kill someone in war. Apparently, it's legal and ethical for a person to kill another if they are holding a gun and pull the trigger, as opposed to doing it remotely from a control room. Where is this book of rules? It sure would be helpful to know.

So, this is a short Fairly Philosophical Friday. I'm tired. I got other things on my mind. Perhaps that will be a blog post of it's own. Someday.