What a defense that is to a war crime.
That's the point. War crimes divide behavior into two categories, in the "heat of battle" and not in the heat of battle.
The list of things that you are allowed under international law to get away with in the heat of battle that you would normally be shot for is quite large.
Often times, heat of battle is the only real defense you have. The Nuremburg trials removed a lot of the "I was following orders" types of defenses...