Suh, Kyungwon2026-01-022026-01-021354-0661ORCID:/0000-0002-7029-0117/work/194749920https://hdl.handle.net/1885/733802417What military and diplomatic instruments are effective for the United States to reassure its allies that their security will be protected as promised? Despite its importance for alliance management, our understanding of alliance reassurance remains incomplete. Specifically, existing research rarely uses direct measures of reassurance outcomes, and pays limited attention to exploring the effectiveness of different toolkits for alliance reassurance. Using a survey experiment with South Korean citizens, this paper examines the effectiveness of a range of military and diplomatic toolkits for reassuring US allies during foreign policy crises. The results demonstrate that conventional and nuclear signals, as well as diplomatic statements of support, significantly assuage the security concerns of South Korean citizens by increasing the perceived safety of South Korea from North Korean military aggression. However, Washington’s diplomatic proposals toward North Korea do not produce similar effects. These findings have implications for debates on interstate signaling and alliance management.The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research is financially supported by the Charles Koch Foundation\u2019s Summer Foreign Policy Fellowship and the Andrew Berlin Family National Security Research Fund from Syracuse University\u2019s Institute for Security Policy and Law.27enPublisher Copyright: © The Author(s) 2025. This article is distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 License (https://creativecommons.org/licenses/by-nc/4.0/) which permits non-commercial use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access page (https://us.sagepub.com/en-us/nam/open-access-at-sage).allianceCrisis diplomacyforeign policyInternational Relationsnuclear diplomacyreassurance2025-06-2610.1177/13540661251349606105010350719