Right, but the war has to happen somewhere, generally in some country. That country's national laws against mass murder would then apply to both sides of the conflict, except that they're trumped by international law that applies to governments. If you're an Italian and you commit murder in the US, as an individual you're subject to American law against murder. But if you're the Italian government and you wage war on the US, the US laws against murder are superseded by the international legality of war. Of course, legalism goes out the window when you're being shot at, in any case.