Research Questions To what extent do steady-state overseas deployments of U.S. forces contribute to interstate deterrence? Do they make militarized disputes or outright wars launched by potential U.S. adversaries more or less likely? If U.S. steady-state forward posture is inadequate to deter an international crisis, can rapid deployments of U.S. forces to the region prevent further escalation of the crisis? Can such crisis deployments secure better bargaining outcomes? For both steady-state and crisis deterrence, do different types of forces produce different effects?

       In the aftermath of Russian military aggression against Ukraine in 2014, and with increasing tensions in the U.S.-China strategic competition, the question of whether U.S. overseas military presence can enhance deterrence remains central. At the same time, U.S. overseas military commitments are increasingly coming under question at home, both among the public at large and among many foreign- and defense-policy elites.

       This report provides empirical evidence on the deterrent effects of U.S. overseas military forces. It investigates how these effects might vary by the type, size, and location of these forces. The analyses provide the clearest evidence in support of the deterrent effect of heavy ground forces, particularly when deployed near but not directly bordering potential adversaries. The analyses show that the more mobile forces are, the less evidence there is of a deterrent effect. The report also assesses the effects of the deployment of U.S. forces during crises, finding that, although they can help avoid escalation and maintain the status quo, they are not necessarily well suited to altering the long-term strategic dynamics between states.

       Key Findings Heavy ground forces are the most likely to enhance deterrence, and crisis deployments may prevent escalation but do not improve partners' leverage The results of the analysis provide consistent evidence for the deterrent effects of heavy ground forces and air defense capabilities, especially when deployed in the general theater of interest but not necessarily on the front lines of a potential conflict. The more mobile forces are, the less evidence there is that they deter. This is possibly because mobile forces represent a lesser degree of high-level or long-term U.S. commitment or possibly because measuring their effects is more difficult. When U.S. forces, particularly light ground forces, are stationed inside the borders of the ally or partner to be defended rather than in nearby states in the broader theater, they are associated with an increased likelihood of militarized disputes. Analysis shows that, when the United States has surged forces forward in an international crisis, there has been a large decline in the incidence of major clashes or war. Deployments of ground and air forces, in particular, were associated with an extremely low incidence of further escalation. Countries supported by U.S. crisis deployments appear no more likely to achieve their strategic goals in a crisis than countries that do not enjoy such support. These results suggest that U.S. crisis deployments can help maintain the status quo at reduced risk of war, but they do not readily translate into bargaining leverage or improved long-term positions for partner states.

       Recommendations This study highlights the importance of considering the type and location of U.S. forces when deciding how to design U.S. overseas deployments to enhance their deterrent value. The general patterns in this study suggest that, in the average case, heavier ground forces and those deployed near, but not directly bordering, potential adversaries may be most likely to reduce the risk of conflict. However, the dynamics of individual situations may differ, and policymakers will need to carefully analyze each situation before deciding on the appropriate approach. This study suggests that U.S. forces rapidly sent to a crisis may help to limit the risk of further escalation. However, policymakers should not expect this type of quick, limited effort to more fundamentally alter the dynamics between the parties that prompted the crisis, for which different types of engagement may be required.

       Table of Contents Chapter One

       Introduction

       Chapter Two

       Debates over the Deterrent Impact of U.S. Forces

       Chapter Three

       General Deterrence

       Chapter Four

       Crisis Deterrence

       Chapter Five

       1961 Berlin Crisis

       Chapter Six

       Operation Vigilant Warrior

       Chapter Seven

       Conclusion and Policy Implications

       Appendix A

       General Deterrence Models

       Appendix B

       Crisis Deterrence Models

       Research conducted by RAND Army Research Division

       This research was sponsored by the Office of the Deputy Chief of Staff for Operations, Plans, and Training (G-3/5/7), U.S. Army and conducted by the Strategy, Doctrine, and Resources Program within the RAND Arroyo Center.

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