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Research Questions

  1. Why does the United States choose coalitions when conducting military interventions?
  2. Why do states join U.S. military coalitions?
  3. What makes military coalitions successful at achieving its political objectives?

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.

Key Findings

  • 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.

Recommendations

  • When it comes to building coalitions, establishing strong ties prior to a conflict or crisis can be beneficial. The Army should seek to build and strengthen these ties with potential partners along political, military, economic, and cultural dimensions.
  • There is some evidence that states seeking greater international status or influence might be primed for recruitment to a coalition, even if they are initially resistant. When forming coalitions, the United States should consider whether there are status-seeking states that might be willing to join without explicit or implicit payments or side payments.
  • Intergovernmental organization sponsorship or leadership might not be required for the formation of strong coalitions, but it can increase the willingness of potential contributors to join. Building and working through NATO or another such organization can help the United States successfully build coalitions for military interventions.
  • Experience with fighting as a coalition can support success in achieving U.S. political objectives. The U.S. Army should continue efforts to train and exercise with potential future partners but emphasize innovation over routine.

Table of Contents

  • 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

Research conducted by

The research described in this report was sponsored by the Office of the Deputy Chief of Staff, G-3/5/7, U.S. Army and conducted by the Strategy, Doctrine, and Resources Program with the RAND Arroyo Center.

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