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• Entanglement dynamics contributed to, but were not the only cause of, U.S. involvement in wars in Korea, Vietnam, and Libya and in two conflicts short of war in the Taiwan Strait.
• Entanglement dynamics in these cases involved a U.S. desire to maintain its reputation with allies and adversaries for upholding its commitments or a U.S. willingness to take on allies' interests as its own.
• More research is needed on how prevalent and consequential entanglement dynamics are in U.S. decisionmaking.
• Scholars have not identified any cases of U.S. entrapment in war, in which the United States fought to defend an ally or partner that risked conflict because a U.S. commitment emboldened it to behave aggressively.
• The United States has allied with states that it believed posed entrapment risks, but it sought to minimize these risks through conditional alliance terms.
• Globally, states in conditional alliances have generally been less likely to initiate conflict, but U.S. alliances could still lead individual states to adopt policies that risk conflict.
• The United States has attempted to restrain allies and partners from initiating conflict in the past by leveraging military and economic aid, and it has had both successes and failures.

The Biden administration has made strengthening U.S. alliances and partnerships a core element of its foreign policy. But some analysts and policymakers have raised concerns about the costs and risks associated with these security relationships, arguing that they cause the United States to adopt its partners' interests as its own, incentivize U.S. allies and partners to engage in reckless behaviors that make conflict more likely, and risk dragging the United States into conflict to protect its reputation for upholding commitments. Other strategists dismiss these concerns. They contend that the United States avoids entanglement by keeping its own interests in mind and restraining its allies and partners from engaging in risky behavior.

In this report—the second in a series on the security and economic trade-offs associated with competing visions for U.S. grand strategy—RAND Corporation researchers assess the evidence for these competing claims by examining and synthesizing the existing empirical literature. They consider one aspect of the larger debate about the future of U.S. security relationships: whether they entangle the United States in wars contrary to its direct interests. This report summarizes the existing research on these questions for U.S. policymakers and offers researchers recommendations on where more research is needed to inform this debate.

• Chapter One

  Introduction

• Chapter Two

  Types of U.S. Security Relationships

• Chapter Three

  Defining Entanglement

• Chapter Four

  Competing Claims About Security Relationships and Entanglement

• Chapter Five

  Does the United States Try to Mitigate Entanglement Risks When Forming Alliances and Partnerships?

• Chapter Six

  How Do U.S. Security Relationships Change Ally and Partner Incentives and Conflict Behavior?

• Chapter Seven

  Does the United States Ultimately Become Entangled in Conflict?

• Chapter Eight

  Findings and Conclusion