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Concern over adversary interventions should be tempered — for now

• Overall, adversary military interventions, in number and in scale, remain far below the levels that the United States had to contend with during the Cold War.
• Several factors could contribute to a shift toward substantially more-aggressive and larger-scale interventions, including intensification of U.S. rivalries with key adversaries (e.g., Russia or China), adversary perceptions of the threats posed by U.S. actions, or dramatic domestic changes in China or Iran that alter how these adversaries think about and use their military forces.

Intervention signposts should be prioritized

• Adversaries are most likely to intervene in response to threats to interests in their home regions, including through military interventions involving combat.
• Analysts may benefit most from watching for evidence of a shift in the regional balance of power or change to the status quo that threatens the adversary's influence or national status.

Of the three adversaries considered, China represents the greatest potential risk to U.S. interests if geopolitical dynamics or shifting national interests were to change its military intervention policy

• China has greater resources and, in some areas, capabilities than Russia or any other U.S. adversary. It also has an expanding set of strategic interests and ambitions outside its home region.
• There are many scenarios that could lead to an increased frequency of Chinese military interventions. Such a shift in Chinese decisionmaking could occur following a sharp deterioration in U.S.-Chinese relations, which would place the two states on opposite sides of an intense militarized rivalry.

Concern about the potential for military interventions by U.S. adversaries to affect U.S. interests has risen over the past decade, driven by high-profile interventions, such as the Russian missions in Ukraine and Syria, Iranian activity in Iraq and Syria, and expanding Chinese military activity in Africa. Despite these concerns, relatively little is known about the intervention behavior of these countries.

There are many reasons for U.S. policymakers to be concerned about the interventions of adversaries. First, adversary interventions might pursue outcomes that undermine U.S. interests. Second, adversary interventions might affect the activities and objectives of U.S. forces when they intervene in the same places. Finally, adversary interventions may directly threaten U.S. forces or U.S. allies.

In this report, the authors explore where, how, and how often U.S. adversaries (specifically, Russia, China, and Iran) have intervened militarily since 1946 and identify why these adversaries have initiated military interventions and why they might do so in the future. Three companion reports consider Chinese, Russian, and Iranian military intervention behavior. The insights and signposts identified can inform U.S. decisions about military posture, partnerships, and investments.

• Chapter One

  Introduction

• Chapter Two

  Why Do States Intervene? A Summary of Past Research

• Chapter Three

  A Quantitative Look at Adversary Military Interventions

• Chapter Four

  Summary of Factors Driving Adversary Interventions

• Chapter Five

  Summary, Signposts, and Implications

• Appendix A

  Full List of Adversary Intervention Cases

• Appendix B

  Coding of Adversary Case Studies

• Appendix C

  Key Factors for Other U.S. Adversaries