Surprisingly enough, firing on another nation's armed forces is not generally defined as 'a war crime'; which is a contravention of the Geneva convention protocols and/or the 'child' international regulations. Even if there's no legal declaration of war it's considered
an act of war, but that's a very different thing. Starting WWIII might legitimately be considered 'a crime against humanity' considering how many people it would eventually kill (more than likely 'all of them' in fact) but again, that's a different thing.
If you're thinking about the whole 'I was only following orders' thing with the Nuremberg trials, you're right insofar as you can't get out of a war crime by saying 'I was told to do it' and ordering a war crime to be committed is considered the same as actually doing it. But shooting someone who isn't shooting at you isn't 'a war crime' just because you're a soldier when you do it.