UK Nuclear deterrence policy: an unlawful threat of force

ABSTRACT This article explores multiple ways in which the unlawfulness of a threat, under Article 2(4), can result from the threatened force being unlawful under another body of law. It concludes: (a) deterrence is a threat; (b) a threat is unlawful if use of the threatened force would be unlawful; (c) the only possible exception to the general rule that use of nuclear weapons would be unlawful is an extreme circumstance of self-defence; (d) use of nuclear weapons in a belligerent reprisal would be unlawful; and so (e) two specific aspects of UK policy are unlawful: the refusal to rule out first use, and the possibility of low level, high power use. Possible strategies to hold the UK to account are considered, and paragraphs 47–8 of the ICJ's Nuclear Weapons advisory opinion are dissected in an appendix. Despite the UK focus, the analysis and conclusions are relevant to other states.