That is indeed a problem. The "law of war" as we understand it was written by Northern European gentlemen in the late 1800s and very early 1900s, and embodied largely in the Hague Conventions of 1899 and 1907. Mostly designed to limit warfare to large, formal armies. They had little understanding of, and no use for, unconventional warfare.
The Geneva Conventions, while important, did little or nothing to change the mindset.
