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• The United States is more likely to rely on military coalitions when operational demands are expected to be high and when there is a need to build international legitimacy.
• Although issues of access and logistical support also factor into coalition decisions, they are typically not sufficient to drive the decision to form a coalition on their own.
• The United States often prefers to act unilaterally when the immediate threat is high, but operational and legitimacy considerations mean that, even in high-threat cases, the United States sometimes has to rely on coalitions.
• States with close ties to the United States are more likely (but not guaranteed) to join coalitions. States without close ties might join for other reasons.
• States are more likely to join coalitions in their home region.
• Shared interests are associated with a higher likelihood of joining a coalition, but states could also enter to pursue their own objectives.
• Sponsorship by an intergovernmental organization, such as NATO, appears to increase states' willingness to join a coalition.
• States sometimes join coalitions to advance their status or reputation.
• Coalitions can support success in achieving objectives, but there is no clear trend in the data.
• Having some shared interest does seem to be associated with a higher likelihood of achieving U.S. objectives; shared threats seem especially important to coalition outcomes.
• Experience with partners in the past provides some benefit but cannot overcome key challenges when it comes to coalition performance.
• Smaller coalitions can operate more efficiently, but sometimes larger coalitions are needed for legitimacy or burden-sharing.

The decision to use a military intervention to achieve a political goal is inherently risky. To offset some of these risks, states sometimes seek to build coalitions made up of partner states that have similar objectives. This report uses quantitative analysis and a series of qualitative case studies to identify and describe factors that seem to be associated with U.S. decisions to use coalitions for military interventions, factors that drive partner states to join such coalitions, and factors that shape the success of military coalitions. The findings indicate that the United States relies on coalitions when operational demands are high and to build international legitimacy for military action. Partners states are most likely to join U.S. coalitions when they have close ties with the United States, when the precipitating crisis is in their home region, when they seek to advance their international standing, and when the coalition has support from an intergovernmental organization. As the United States faces more significant threats from near peer competitors, it may need to rely on partners more heavily and can leverage the insights in this report to construct strong and durable coalitions.

• Chapter One

  Introduction

• Chapter Two

  Learning from Past Research on Coalition Interventions

• Chapter Three

  Data and Methodology

• Chapter Four

  When Does the United States Use Coalitions?

• Chapter Five

  Coalition Partners: Who Joins and Why?

• Chapter Six

  What Makes Coalition Interventions Successful?

• Chapter Seven

  Conclusions

• Appendix A

  Codebook for the RAND U.S. Coalition Intervention Dataset

• Appendix B

  Lists of U.S. Coalition and Noncoalition Military Interventions (1946–2018)

• Appendix C

  Technical Appendix