What Is a "War Crime"?

Basically a violation of the Laws of War or Law of Land Warefare which is pretty much the Hague Conventions and Geneva Conventions. You can get some details in DA FM 27-10.

If you have the chance, watch the movie Tokyo trial it talk about the three definitions of war crimes based on the 1946 and 1948 Tokyo trials on Japanese politicians and Military. It is available on Netflix it's awesome