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• U.S. military interventions can advance U.S. interests, but they can also be highly costly or counterproductive when used in the wrong circumstances. Our review of the historical record highlights the importance of a case-by-case assessment of intervention circumstances.
• The advisability of an intervention is likely to be affected by its potential to affect the local balance of power in a crisis or conflict. In some cases, the intervention could be necessary to shift the balance of power in a way that makes a more favorable outcome possible. Other factors, such as local, popular reaction and the actions of potential third-party spoilers, are also essential to consider.
• Forgoing an intervention might be preferable, even when clear U.S. interests are involved, when any plausible U.S. intervention force might be insufficient to alter the outcome, or when a favorable outcome is likely already. Historically, more nonintervention cases resulted in improved outcomes for U.S. interests than resulted in deteriorated outcomes.
• Interventions early in a conflict or crisis tended to advance U.S. interests more than those that occurred later, though this finding varied depending on the context, and there were clear exceptions.
• Larger intervention forces can help the United States better achieve its objectives in certain circumstances. In other cases, however, larger forces could be ineffective or counterproductive if they can only be deployed with greater delay, if they lead to a backlash among the local population, or if they prompt unwanted escalation on the part of an adversary.

When faced with the outbreak of a war or the onset of an international crisis that affects important U.S. interests, U.S. policymakers must consider how best to respond. One option they may consider is to directly intervene militarily in the war or crisis. In this report, the authors create a framework that can be used to rigorously consider the trade-offs between intervening militarily early in a war or crisis, intervening later, and not intervening at all, as well as the trade-offs involved in decisions regarding the size of the potential intervention force to be employed. This framework can provide a better understanding of the relationships between intervention timing, intervention size, and intervention outcomes to inform future debates about whether and when to undertake a military intervention.

The authors approached these issues in four ways. They conducted (1) an extensive review of academic and policy literature on relevant topics, (2) a quantitative analysis using a database of 286 crises and wars since 1945 in which important U.S. interests were at stake, (3) a set of 45 short, focused case studies, including both interventions and noninterventions, and (4) a set of four in-depth counterfactual illustrations, in which U.S. intervention decisions were altered from historical events to explore the implications. The findings from these four research approaches lead to recommendations regarding the contextual factors that are most essential for policymakers to consider when making intervention decisions.

• Chapter One

  Introduction

• Chapter Two

  What Do We Know? Reviewing Existing Literature

• Chapter Three

  Quantitative Analysis: Studying Trends and Patterns

• Chapter Four

  Lessons from Focused Case Studies

• Chapter Five

  Counterfactuals

• Chapter Six

  Conclusion and Implications

• Appendix A

  List of Cases of Potential U.S. Intervention

• Appendix B

  Counterfactual Case Studies