Most "terrorists" aren't particularly different from regular soldiers when it comes down to it. In the case of Iraq, they quite often will be former regular soldiers anyway.

"Rules of war" do matter. They mean, for example, that both sides refrain from mistreating prisoners, or from recklessly endangering non-combatants, and that people who do things like that, or authorise them, should, at least in principle, be put on trial for it, as putative war criminals. People on either side. (Some chance, of course - but it's a nod in the direction of civilisation that most countries have made.)