The prohibition on the threat of force is typically understood to apply to threats to have unlawful recourse to force (jus ad bellum). Much less attention has been paid to threats to use force in an unlawful manner, i.e., in violation of jus in bello international humanitarian law (IHL) rules.
Neither Article 2(4) of the U.N. Charter nor international law generally prohibits threats to use force in contravention of IHL. But for parties to the 1949 Geneva Conventions or 1977 Additional Protocol I, threatening to use force in blatant or manifest violation of IHL would violate the obligation under Common Article 1 to ensure respect for those treaties.
This has important implications for nuclear deterrence. For parties to Additional Protocol I, threats contemplating blatant or manifest violations of the rules of distinction, proportionality, and precaution would breach the duty to ensure respect for the Protocol.
Even where threats to use force in violation of international law would not be unlawful, there are sound policy reasons not to threaten to use force in violation of IHL. Such threats offer doubtful security benefits and could have the collateral effect of weakening compliance with IHL during the course of actual armed conflicts.