Why reassuring allies is harder than deterring adversaries in extended deterrence: Evidence from US extended deterrence for South Korea

In extended deterrence, a nuclear patron assumes a dual role: reassuring its clients while deterring adversaries. Policymakers and scholars widely recognise that for a patron, reassurance is more challenging than deterrence. Why? What makes reassurance harder? I argue that a patron’s deliberate introduction of ambiguity into its extended deterrence commitments – designed to reduce the risk of entrapment – makes reassurance more difficult. This ambiguity makes a patron’s security umbrella appear unreliable to clients, thereby producing uniformly negative effects on reassurance. Conversely, a patron’s ambiguous commitments have a net-neutral effect on adversaries, sustaining a consistent deterrent effect. Ultimately, this dual effect of ambiguity renders reassurance a more difficult task than deterrence. I illustrate the plausibility of this argument by examining how ambiguities in US extended deterrence have been perceived by South Korea and North Korea.