As I understand it, the distinction goes back to the Geneva convention and similar rules of "just war" that aim to minimize harm to civilians. Specifically, if

As I understand it, the distinction goes back to the Geneva convention and similar rules of "just war" that aim to minimize harm to civilians. Specifically, if you openly declare war, wear uniforms, carry weapons openly and follow other rules of war, your opponents can blow you up while leaving civilians alone (and you can do the same to them). Then you've just got a regular war going on.