Research Center for Nuclear Weapons Abolition

Throughout the nuclear age, states have made a wide array of threats to use nuclear weapons. There is, however, often little clarity as to whether such threats are legal or illegal under international law. This article is the second in a two-part series, and in this piece we examine how two specific sets of international legal rules apply to select examples of past nuclear threats. In particular we analyse the legality of certain threats under the jus ad bellum regime of international law that regulates recourse to war between states, before turning to consider specific threat examples in the context of the jus in bello regime, which applies to regulate the conduct of hostilities during an armed conflict. Throughout the article, we identify a number of complexities and deficiencies in the ways that the rules of jus ad bellum and jus in bello apply to nuclear threats in practice.

Keywords: Nuclear weapons; international law; threat of force; nuclear threat

Ulrich Kühn is Director of the Arms Control and Emerging Technologies Program at IFSH, University of Hamburg, and a Non-Resident Scholar with the Nuclear Policy Program, Carnegie Endowment for International Peace.

Heather Williams is the Director of the Project on Nuclear Issues at the Center for Strategic and International Studies. She is also an associate fellow with the Project on Managing the Atom, Harvard Belfer Center.