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
- "Strategies of Extended Deterrence: How States Provide the Security Umbrella," Security Studies 30, no. 5 (December 2021): 761-796.
- "Upgrading the Bomb: Why and How the US Provides Advanced Nuclear Assistance to Junior Allies," The Chinese Journal of International Politics (Forthcoming)
Why and how does the US provide advanced nuclear assistance to junior nuclear allies? Existing nuclear literature downplays the possibility that the US strategically employs the technical assistance option as a post-proliferation toolkit to control allied nuclear proliferation. Addressing the first question—why—I argue that the US provides advanced nuclear assistance to junior allies to inhibit the latter’s growing tendency towards unilateralism after nuclear acquisition, meanwhile inducing greater allied nuclear coordination. By forging advanced nuclear partnerships, the US seeks simultaneously to draw junior allies closer to the American nuclear leadership and establish coordinated nuclear strike plans and joint nuclear targeting. Addressing the second question—how—I claim that the US provides advanced nuclear assistance at a protracted pace and in a restricted manner, calculated to exert influence over recipient junior allies for as long as possible. To provide the plausibility probe of my argument, I explore two historical cases: The US’s strategic advanced nuclear assistance to the UK and to France. I conclude by identifying avenues for future study and the policy implications for China’s potential response to North Korea’s nuclear sophistication.
Selected Working Papers
- "Deter Together or Deter Separately?: Time Horizons and Peacetime Alliance Cohesion of the US-Japan and the US-ROK Alliances" (Revise & Resubmit)
What explains the degree of peacetime alliance cohesion? Why are some alliances cohesive about common external threats and others not? I argue that the interaction of allies’ threat-specific time horizons determines whether allies are cohesive or incohesive with each other in deterring common threats. I demonstrate that how allies behave towards common threats differs depending on the length of their threat-specific time horizons (either "short" or "long"). Based on the demonstration, I predict that when the lengths of allies’ time horizons are mutually congruent with each other, alliance cohesion will be high. Conversely, when they are incongruent, alliance cohesion will be low. Subsequently, I provide a novel typology to systematically measure allies’ time horizons. To test my argument, I conduct cross-case analyses of peacetime alliance cohesion regarding the rise of China over the past two decades in the US-Japan and US-Republic of Korea alliances.
- "The Visibility of Extended Security Commitments and Reassurance Effects" (with Jiyoung Ko)
What role does the visibility of extended security commitments play in reassuring a client state? We investigate this question by revisiting the sources of visibility of extended security guarantees. It is generally believed that if a patron’s extended deterrent assets are more visible to a client’s foreign policy elites and populations, such visibility will greatly reassure the client. Conventional wisdom suggests that the visibility of alliance commitments is mainly determined by the size, permeance, and location of the patron’s foreign troop deployment. Challenging this view, we claim that the reassurance effect emanated from the visibility is driven more by the diversity of extended deterrent assets rather than their size and the location. We also argue that once a patron permanently deploys its troops to a client, its permanence paradoxically reduces reassuring effects by leading allied populations to become accustomed to the patron’s military footprint. We conduct original survey experiments to evaluate South Korean and Japanese respondents’ attitudes towards the different types of US military presence. This research contributes to a deepened understanding of the relationship between the visibility of extended security commitments and reassurance.
- 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
- "Strategies of Extended Deterrence: How States Provide the Security Umbrella," Security Studies 30, no. 5 (December 2021): 761-796.
- "Upgrading the Bomb: Why and How the US Provides Advanced Nuclear Assistance to Junior Allies," The Chinese Journal of International Politics (Forthcoming)
Why and how does the US provide advanced nuclear assistance to junior nuclear allies? Existing nuclear literature downplays the possibility that the US strategically employs the technical assistance option as a post-proliferation toolkit to control allied nuclear proliferation. Addressing the first question—why—I argue that the US provides advanced nuclear assistance to junior allies to inhibit the latter’s growing tendency towards unilateralism after nuclear acquisition, meanwhile inducing greater allied nuclear coordination. By forging advanced nuclear partnerships, the US seeks simultaneously to draw junior allies closer to the American nuclear leadership and establish coordinated nuclear strike plans and joint nuclear targeting. Addressing the second question—how—I claim that the US provides advanced nuclear assistance at a protracted pace and in a restricted manner, calculated to exert influence over recipient junior allies for as long as possible. To provide the plausibility probe of my argument, I explore two historical cases: The US’s strategic advanced nuclear assistance to the UK and to France. I conclude by identifying avenues for future study and the policy implications for China’s potential response to North Korea’s nuclear sophistication.
Selected Working Papers
- "Deter Together or Deter Separately?: Time Horizons and Peacetime Alliance Cohesion of the US-Japan and the US-ROK Alliances" (Revise & Resubmit)
What explains the degree of peacetime alliance cohesion? Why are some alliances cohesive about common external threats and others not? I argue that the interaction of allies’ threat-specific time horizons determines whether allies are cohesive or incohesive with each other in deterring common threats. I demonstrate that how allies behave towards common threats differs depending on the length of their threat-specific time horizons (either "short" or "long"). Based on the demonstration, I predict that when the lengths of allies’ time horizons are mutually congruent with each other, alliance cohesion will be high. Conversely, when they are incongruent, alliance cohesion will be low. Subsequently, I provide a novel typology to systematically measure allies’ time horizons. To test my argument, I conduct cross-case analyses of peacetime alliance cohesion regarding the rise of China over the past two decades in the US-Japan and US-Republic of Korea alliances.
- "The Visibility of Extended Security Commitments and Reassurance Effects" (with Jiyoung Ko)
What role does the visibility of extended security commitments play in reassuring a client state? We investigate this question by revisiting the sources of visibility of extended security guarantees. It is generally believed that if a patron’s extended deterrent assets are more visible to a client’s foreign policy elites and populations, such visibility will greatly reassure the client. Conventional wisdom suggests that the visibility of alliance commitments is mainly determined by the size, permeance, and location of the patron’s foreign troop deployment. Challenging this view, we claim that the reassurance effect emanated from the visibility is driven more by the diversity of extended deterrent assets rather than their size and the location. We also argue that once a patron permanently deploys its troops to a client, its permanence paradoxically reduces reassuring effects by leading allied populations to become accustomed to the patron’s military footprint. We conduct original survey experiments to evaluate South Korean and Japanese respondents’ attitudes towards the different types of US military presence. This research contributes to a deepened understanding of the relationship between the visibility of extended security commitments and reassurance.
* Header image: NATO Headquarters in Brussels, Belgium. (Photo from NATO website)