The only "law of war" that we need is one that states that war is not allowed, period. If you're an aggressor, you are breaking the "law of war".
That sounds really great, but I doubt it would work well in practice. The colonies would still be under the Crown, who never lost power. France would be under control of Louis XXXXII, the South would still run on slavery, etc. Without war and revolution, how would despotic regimes ever end?
Natural laws have no pity.