The Law of the Earth answers the question of war with one principle: force used in defense against harm is the only force that can ever be legitimate. All non-defensive force is unlawful.
When two sides each claim to be defending themselves, the Law gives a method: trace the chain of force back to its origin, the first act of force that was not in defense of anything. That act is illegitimate, and so is all force that protects it. And even defensive force must be proportionate, doing no greater harm than the harm it answers.
No one can hide behind a nation. No act has ever been performed by a government; every act is performed by a person, who is accountable for it. Ordering a crime carries the same accountability as committing it, and so does voting for it. "I was just following orders" is not an excuse. (Title 4.)
Nuclear, chemical, and biological weapons are contraband, and building or researching them is unlawful everywhere. (Title 3.) And because vengeance is always unlawful (Title 5), no war fought to punish or retaliate can ever be lawful.