Extracts from the Rome Statute of the International Criminal Court
Article 8: War Crimes
1. The Court shall have jurisdiction in respect of war crimes in particular
when committed as part of a plan or policy or as part of a large-scale
commission of such crimes.
2. For the purpose of this Statute, “war crimes” means:
… (b) Other serious violations of the laws and customs applicable in
international armed conflict, within the established framework of international law, namely, any of the following acts:
(iv) Intentionally launching an attack in the knowledge that such attack will cause incidental loss of life or injury to civilians or damage to civilian objects or widespread, long-term and severe damage to the natural environment which would be clearly excessive in relation to the concrete and direct overall military advantage anticipated;
This doesn’t quite get us there. It seems that an overwhelming military necessity might justify the use of nuclear weapons. But in spite of repeated requests, no nuclear-armed state has come up with any imaginable military necessity which could be worth killing on a mass scale, long-lasting damage to the planet’s climate, and a widespread risk to civilised life. It could be argued that a very limited use of a few nuclear nuclear weapons just to “warn off” an enemy would not have these devastating consequences. But this does not take account of an uncontrollable escalation. Neither does it explain why there are thousands of large warheads poised for launch. They must be there for some reason.
499. War Crimes: The term “war crime” is a technical expression for violation of the law of war by any person or persons, military or civilian. Every violation of the law of war is a war crime....
This dates from some time ago, but later revisions still clearly state this principle. It dispels any doubt that violating the Laws of War would be criminal.