We're still bound by the laws of just warfare. (The men who flew the planes into the towers on 9/11 could have said the same thing: "You deserve this; it's biblical.")
As a graduate of the Navy’s Law of War course, that is false. There were four criterion the 9/11 attackers failed to meet a just standard of war.
Secondly, “bound” by International Law has grey areas. Chiefly, it is the victors who apply justice, as the institution of war is messy at best, and that is not even considering the sinfulness of man attempting to put guardrails on the conduct of war.
Third, the biblical view trumps whatever philosophy or devices adopted by man. The Japanese had every opportunity to conclude a peace, but their pride took them to mass destruction. As a heathen country, they earned their wages; “for the wages of sin is death.” Romans 6:23.
It matters what god a nation follows and exults.