International Humanitarian Law (IHL): Nuclear weapons are bound by the principles of IHL, particularly the rules of distinction, proportionality, and necessity. These principles aim to limit the effects of armed conflict on civilians and ensure that military actions are directed at legitimate military targets.
The International Court of Justice (ICJ) Advisory Opinion (1996): The ICJ, in its advisory opinion, stated that the use of nuclear weapons could be unlawful if it violates the principles of IHL, particularly if it causes harm to civilians. However, the Court did not give a definitive ruling on the legality of nuclear weapons in all situations, recognizing the uncertainty about their use in extreme circumstances like self-defense.
In conclusion, the use of nuclear weapons in armed conflicts remains highly controversial and is subject to legal scrutiny. It is considered potentially unlawful under IHL, depending on the circumstances, due to the disproportionate and indiscriminate effects that these weapons cause.