As Michael Kinsley points out, there are thousands and thousands of pages of legal analysis, treaties, definitions and conventions regarding the rules of war. All of these are internationally accepted.
But now, our government is attempting to justify its use of drones in other nations by defining the legal justification for doing so after the fact. That would be akin to an accused murderer drafting laws on homicide after committing the act.
We may have the need to use drones, but that need does not give us the right under current international laws and treaties to do so.