Sole Purpose Doctrine

The sole purpose doctrine narrows the declared role of nuclear weapons to deterring or responding to nuclear use against the state or its allies. It is a more restrictive posture than "calculated ambiguity," under which a state reserves the right to use nuclear weapons in response to a wider range of threats — including large-scale conventional attacks, or chemical and biological weapons use — but it is generally treated as slightly less restrictive than a full no-first-use (NFU) pledge, because it focuses on declaratory purpose rather than an absolute commitment never to strike first.

The concept has been most prominent in U.S. nuclear policy debates. The Obama administration's 2010 Nuclear Posture Review stated a long-term aspiration to make deterrence of nuclear attack the "sole purpose" of U.S. nuclear weapons, but stopped short of adopting the policy outright, citing concerns from allies reliant on extended deterrence. Joe Biden, as a presidential candidate and earlier as vice president, publicly endorsed moving toward a sole purpose declaration; however, the 2022 Nuclear Posture Review released by his administration declined to adopt either sole purpose or NFU, retaining a role for nuclear weapons in deterring non-nuclear strategic attacks.

Allied governments — particularly in NATO, Japan, and South Korea — have historically resisted such shifts, arguing that ambiguity strengthens extended deterrence against conventional and WMD threats from Russia, China, and North Korea. Advocates counter that sole purpose:

• reduces the risk of inadvertent escalation,
• supports nonproliferation commitments under Article VI of the NPT, and
• aligns declaratory policy with the practical implausibility of nuclear first use.

For MUN and policy researchers, sole purpose is best understood as a middle option on a spectrum running from open first-use postures, through calculated ambiguity, sole purpose, and finally no-first-use.