Book Project
- Tailoring Extended Deterrence: How States Provide the Security Umbrella
Nuclear patrons extend deterrence to other non-nuclear clients for 1) deterring potential aggressors from attacking the clients; and 2) assuring the clients of their security. For the dual goals, nuclear patrons have employed different strategies across disparate clients. Besides, their strategy for the same client’s protection has varied over time.
For example, over the last 70 years since the formation of North Atlantic Treaty Organization (NATO), the US security commitment to NATO-Europe has relied on forward-deployed nuclear weapons coupled with large-scale conventional troops in the European continent. Although the US extended deterrence over South Korea has remained at a nuclear-level as well, it underwent a considerable change in the early 1990s. That is, America’s preexisting on-shore nuclear umbrella shifted to an offshore nuclear shield. In contrast, the US extended deterrence to the Philippines during the Cold War was conventional: the US prepositioned robust conventional troops in the Archipelago, and it did not publicly make any nuclear commitment or openly employ nuclear assets for the client’s protection.
In short, the security umbrellas provided have not been one-size-fits-all. Instead, they have been tailored to individual clients’ unique security needs and circumstances. However, we still do not have a systematic framework to understand how nuclear patron states provide a security umbrella for nonnuclear states. What extended deterrence strategies are available to nuclear patrons and what factors determine which strategy they will adopt (strategy adoption)? How are the individual strategies embodied as actual force employment (strategy implementation)? This study aims to answer the questions by identifying the divergent causal paths that lead patrons to develop a distinct extended deterrent posture over time and across clients.
With regards to the question of strategy adoption, I argue that the interplay of two variables—1) the type of threat posed by an enemy to a client and 2) the likelihood of an enemy’s quick victory in the event of war with a client—determines which of four distinct strategies a nuclear patron will adopt: “forward nuclear deployment,” the “nuclear defense pact,” “forward conventional deployment,” and the “conventional defense pact.” Regarding the threat variable, I distinguish between two types of threats—existential and non-existential—determined by the enemy’s claimed goal in conflict with a client. With regards to the quick-victory variable, I distinguish between two possibilities—high and low—determined by the enemy’s surprise and mobile warfare capability vis-à-vis a client.
The type of threat posed by an enemy to a client determines a patron’s required level of security commitment to assure the client—whether nuclear or conventional level. The likelihood of an enemy’s swift overrun of a client affects the location of a patron’s prepositioned forces to deter the enemy—whether forward or rear. The interaction of two dichotomous variables generates a set of extended deterrence strategies, creating four testable hypotheses as follows.
First, when a patron judges that a client faces an existential threat (e.g. a threat of entire political absorption) and an enemy’s quick victory is highly likely, a patron will adopt a “forward nuclear deployment” strategy. Second, when a client is under an existential threat and an enemy’s swift victory seems unlikely, a patron will adopt a “nuclear defense pact” strategy. Third, when a client is faced with merely a non-existential threat (e.g. a threat of territorial annexation) and an enemy’s swift overrun of a client is likely, a patron will adopt a “forward conventional deployment” strategy. Finally, when a client faces a non-existential threat and an enemy’s swift victory seems unlikely, a patron will adopt a “conventional defense pact” strategy.
With regards to the question of strategy implementation, I argue that each strategy is characterized by a unique mix of conventional and nuclear assets prepositioned in either forward or rear areas. “Forward nuclear deployment” is embodied as the combination of on-shore tactical nuclear weapons and substantial conventional shield troops along the frontline. Second, a “nuclear defense pact” is comprised of off-shore strategic nuclear forces coupled with conventional token forces in the rear area of the client’s soil. Third, “forward conventional deployment” is manifested as sizable conventional frontline troops, but an absence of any prepositioned nuclear assets. Lastly, a “conventional defense pact” contains neither conventional nor nuclear forward military presence.
To substantiate my argument, I examine four cases of extended deterrence: 1) US extended deterrence to NATO-Europe, 2) US extended deterrence to South Korea, 3) US extended deterrence to the Philippines, and 4) Soviet extended deterrence to Mongolia.
Peer-reviewed Journal Articles
- "Deter Together or Deter Separately?: Time Horizons and Peacetime Alliance Cohesion of the US-Japan and the US-ROK Alliances," The Pacific Review 37, no. 4 (2024): 747–776.
- "Upgrading the Bomb: Why and How the US Provides Advanced Nuclear Assistance to Junior Allies," The Chinese Journal of International Politics 16, no. 2 (2023): 181–207.
- "Strategies of Extended Deterrence: How States Provide the Security Umbrella," Security Studies 30, no. 5 (2021): 761–796.
Editor-reviewed Journal Articles
- “The Case against Nuclear Sharing in East Asia,” The Washington Quarterly 44, no. 4 (2021), 67–87 (with Joshua Byun).
Selected Working Papers
- "Why Reassuring Allies is Harder than Deterring Adversaries in Extended Deterrence"
A nuclear patron assumes a dual role in extended deterrence: reassuring clients while deterring enemy invasion against them. Throughout history, patrons have frequently encountered difficulties in providing reassurance to allies, even when their efforts to deter enemies have been successful. In this regard, Denis Healey, the UK’s former Secretary of Defense, highlighted the greater difficulty of providing reassurance compared to deterrence in what is called the Healey Theorem. Why is reassuring clients more challenging than deterring enemies in extended deterrence? I propose two original mechanisms to answer this question. First, inherent ambiguities in extended deterrent commitments, pursued by a patron to mitigate the risk of entrapment, generate deterrence-enhancing effects while concurrently weakening reassurance of clients. Second, a patron’s broader spatial thinking aimed at securing strategic flexibility in force deployment aligns with the enemy’s aggression calculus, which is also shaped by expansive spatial considerations. However, the patron’s broader perspective conflicts with the client’s narrower, locally-centered spatial horizons. I test the plausibility of my arguments using the case of US extended deterrence over South Korea and find strong support from empirical evidence.
- Continuity and changes: The effects of Russia’s war against Ukraine on Japanese and South Korean nuclear weapons discourse (with Tongfi Kim, conditionally accepted)
This article examines the continuity and changes in Japanese and South Korean discourses on nuclear weapons after Russia renewed its aggression against Ukraine in February 2022. The public of both countries were shocked by the war, but they had contrasting reactions in terms of their attitude toward nuclear weapons. Although Japan’s national security discourse underwent significant changes, its discourse on nuclear weapons remained largely unchanged. In contrast, South Korea’s nuclear weapons policy discourse, supported by public opinion and further accelerated by the Yoon Suk Yeol administration’s policy preferences, has undergone significant changes since the beginning of the war. Although the presence and absence of norms against nuclear weapons in the two countries appear to offer an easy explanation for the outcomes, the article shows that strategic considerations—the distinct nature of the primary external threats faced by the two states—matter in both cases. Specifically, Japan maintains a distance from nuclear options, focusing on reinforcing its conventional capability to deter non-existential threats from China. In contrast, South Korea engages in active discussions regarding diverse nuclear deterrent options to hedge against existential threats from North Korea.
-"Seeing is Believing? Revisiting the Reassurance Effects of Military Demonstrations in Extended Dterrence" (with Jiyoung Ko)
Do a patron’s military demonstrations help reassure clients? A patron like the United States often engages in military demonstrations (e.g., troop deployments and joint military exercises) to reassure allied audiences of its willingness and capability to protect them. There is a widespread belief that more frequent and consistent military demonstrations lead to greater reassurance effects. However, despite being key locations for U.S. military demonstrations, the United Kingdom and France were not reassured by the U.S. nuclear umbrella and eventually developed their own nuclear weapons. Similarly, West Germany and South Korea pursued independent nuclear armament despite significant U.S. military demonstrations, only abandoning their nuclear ambitions under U.S. coercive pressure. To address this empirical mismatch, we argue that a theory capturing the nuanced dynamics of military demonstrations is needed. An allied audience’s interpretation of the visible signals from a patron’s military demonstration is fundamentally a psychological and cognitive process. By drawing on insights from psychology, advertising, and media studies, we propose three revised perspectives on military demonstrations. We conducted a small pilot survey experiment in South Korea to test our claims. This research contributes to a deeper understanding of the relationship between military demonstrations and their reassurance effects.
Op-ed Articles
- "The Prospects for North Korea-Russia Nuclear Cooperation" The Diplomat, November 10, 2023.
- Tailoring Extended Deterrence: How States Provide the Security Umbrella
Nuclear patrons extend deterrence to other non-nuclear clients for 1) deterring potential aggressors from attacking the clients; and 2) assuring the clients of their security. For the dual goals, nuclear patrons have employed different strategies across disparate clients. Besides, their strategy for the same client’s protection has varied over time.
For example, over the last 70 years since the formation of North Atlantic Treaty Organization (NATO), the US security commitment to NATO-Europe has relied on forward-deployed nuclear weapons coupled with large-scale conventional troops in the European continent. Although the US extended deterrence over South Korea has remained at a nuclear-level as well, it underwent a considerable change in the early 1990s. That is, America’s preexisting on-shore nuclear umbrella shifted to an offshore nuclear shield. In contrast, the US extended deterrence to the Philippines during the Cold War was conventional: the US prepositioned robust conventional troops in the Archipelago, and it did not publicly make any nuclear commitment or openly employ nuclear assets for the client’s protection.
In short, the security umbrellas provided have not been one-size-fits-all. Instead, they have been tailored to individual clients’ unique security needs and circumstances. However, we still do not have a systematic framework to understand how nuclear patron states provide a security umbrella for nonnuclear states. What extended deterrence strategies are available to nuclear patrons and what factors determine which strategy they will adopt (strategy adoption)? How are the individual strategies embodied as actual force employment (strategy implementation)? This study aims to answer the questions by identifying the divergent causal paths that lead patrons to develop a distinct extended deterrent posture over time and across clients.
With regards to the question of strategy adoption, I argue that the interplay of two variables—1) the type of threat posed by an enemy to a client and 2) the likelihood of an enemy’s quick victory in the event of war with a client—determines which of four distinct strategies a nuclear patron will adopt: “forward nuclear deployment,” the “nuclear defense pact,” “forward conventional deployment,” and the “conventional defense pact.” Regarding the threat variable, I distinguish between two types of threats—existential and non-existential—determined by the enemy’s claimed goal in conflict with a client. With regards to the quick-victory variable, I distinguish between two possibilities—high and low—determined by the enemy’s surprise and mobile warfare capability vis-à-vis a client.
The type of threat posed by an enemy to a client determines a patron’s required level of security commitment to assure the client—whether nuclear or conventional level. The likelihood of an enemy’s swift overrun of a client affects the location of a patron’s prepositioned forces to deter the enemy—whether forward or rear. The interaction of two dichotomous variables generates a set of extended deterrence strategies, creating four testable hypotheses as follows.
First, when a patron judges that a client faces an existential threat (e.g. a threat of entire political absorption) and an enemy’s quick victory is highly likely, a patron will adopt a “forward nuclear deployment” strategy. Second, when a client is under an existential threat and an enemy’s swift victory seems unlikely, a patron will adopt a “nuclear defense pact” strategy. Third, when a client is faced with merely a non-existential threat (e.g. a threat of territorial annexation) and an enemy’s swift overrun of a client is likely, a patron will adopt a “forward conventional deployment” strategy. Finally, when a client faces a non-existential threat and an enemy’s swift victory seems unlikely, a patron will adopt a “conventional defense pact” strategy.
With regards to the question of strategy implementation, I argue that each strategy is characterized by a unique mix of conventional and nuclear assets prepositioned in either forward or rear areas. “Forward nuclear deployment” is embodied as the combination of on-shore tactical nuclear weapons and substantial conventional shield troops along the frontline. Second, a “nuclear defense pact” is comprised of off-shore strategic nuclear forces coupled with conventional token forces in the rear area of the client’s soil. Third, “forward conventional deployment” is manifested as sizable conventional frontline troops, but an absence of any prepositioned nuclear assets. Lastly, a “conventional defense pact” contains neither conventional nor nuclear forward military presence.
To substantiate my argument, I examine four cases of extended deterrence: 1) US extended deterrence to NATO-Europe, 2) US extended deterrence to South Korea, 3) US extended deterrence to the Philippines, and 4) Soviet extended deterrence to Mongolia.
Peer-reviewed Journal Articles
- "Deter Together or Deter Separately?: Time Horizons and Peacetime Alliance Cohesion of the US-Japan and the US-ROK Alliances," The Pacific Review 37, no. 4 (2024): 747–776.
- "Upgrading the Bomb: Why and How the US Provides Advanced Nuclear Assistance to Junior Allies," The Chinese Journal of International Politics 16, no. 2 (2023): 181–207.
- "Strategies of Extended Deterrence: How States Provide the Security Umbrella," Security Studies 30, no. 5 (2021): 761–796.
Editor-reviewed Journal Articles
- “The Case against Nuclear Sharing in East Asia,” The Washington Quarterly 44, no. 4 (2021), 67–87 (with Joshua Byun).
Selected Working Papers
- "Why Reassuring Allies is Harder than Deterring Adversaries in Extended Deterrence"
A nuclear patron assumes a dual role in extended deterrence: reassuring clients while deterring enemy invasion against them. Throughout history, patrons have frequently encountered difficulties in providing reassurance to allies, even when their efforts to deter enemies have been successful. In this regard, Denis Healey, the UK’s former Secretary of Defense, highlighted the greater difficulty of providing reassurance compared to deterrence in what is called the Healey Theorem. Why is reassuring clients more challenging than deterring enemies in extended deterrence? I propose two original mechanisms to answer this question. First, inherent ambiguities in extended deterrent commitments, pursued by a patron to mitigate the risk of entrapment, generate deterrence-enhancing effects while concurrently weakening reassurance of clients. Second, a patron’s broader spatial thinking aimed at securing strategic flexibility in force deployment aligns with the enemy’s aggression calculus, which is also shaped by expansive spatial considerations. However, the patron’s broader perspective conflicts with the client’s narrower, locally-centered spatial horizons. I test the plausibility of my arguments using the case of US extended deterrence over South Korea and find strong support from empirical evidence.
- Continuity and changes: The effects of Russia’s war against Ukraine on Japanese and South Korean nuclear weapons discourse (with Tongfi Kim, conditionally accepted)
This article examines the continuity and changes in Japanese and South Korean discourses on nuclear weapons after Russia renewed its aggression against Ukraine in February 2022. The public of both countries were shocked by the war, but they had contrasting reactions in terms of their attitude toward nuclear weapons. Although Japan’s national security discourse underwent significant changes, its discourse on nuclear weapons remained largely unchanged. In contrast, South Korea’s nuclear weapons policy discourse, supported by public opinion and further accelerated by the Yoon Suk Yeol administration’s policy preferences, has undergone significant changes since the beginning of the war. Although the presence and absence of norms against nuclear weapons in the two countries appear to offer an easy explanation for the outcomes, the article shows that strategic considerations—the distinct nature of the primary external threats faced by the two states—matter in both cases. Specifically, Japan maintains a distance from nuclear options, focusing on reinforcing its conventional capability to deter non-existential threats from China. In contrast, South Korea engages in active discussions regarding diverse nuclear deterrent options to hedge against existential threats from North Korea.
-"Seeing is Believing? Revisiting the Reassurance Effects of Military Demonstrations in Extended Dterrence" (with Jiyoung Ko)
Do a patron’s military demonstrations help reassure clients? A patron like the United States often engages in military demonstrations (e.g., troop deployments and joint military exercises) to reassure allied audiences of its willingness and capability to protect them. There is a widespread belief that more frequent and consistent military demonstrations lead to greater reassurance effects. However, despite being key locations for U.S. military demonstrations, the United Kingdom and France were not reassured by the U.S. nuclear umbrella and eventually developed their own nuclear weapons. Similarly, West Germany and South Korea pursued independent nuclear armament despite significant U.S. military demonstrations, only abandoning their nuclear ambitions under U.S. coercive pressure. To address this empirical mismatch, we argue that a theory capturing the nuanced dynamics of military demonstrations is needed. An allied audience’s interpretation of the visible signals from a patron’s military demonstration is fundamentally a psychological and cognitive process. By drawing on insights from psychology, advertising, and media studies, we propose three revised perspectives on military demonstrations. We conducted a small pilot survey experiment in South Korea to test our claims. This research contributes to a deeper understanding of the relationship between military demonstrations and their reassurance effects.
Op-ed Articles
- "The Prospects for North Korea-Russia Nuclear Cooperation" The Diplomat, November 10, 2023.
* Header image: NATO Headquarters in Brussels, Belgium. (Photo from NATO website)