Cover: Assessing Trade-Offs in U.S. Military Intervention Decisions

Assessing Trade-Offs in U.S. Military Intervention Decisions

Whether, When, and with What Size Force to Intervene

Published Dec 13, 2021

by Bryan Frederick, Jennifer Kavanagh, Stephanie Pezard, Alexandra Stark, Nathan Chandler, James Hoobler, Jooeun Kim


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Research Synopsis

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Research Questions

  1. Under what circumstances should the United States undertake a military intervention?
  2. When is it better to intervene early in a conflict or crisis rather than later?
  3. When is it better to intervene with larger forces rather than smaller forces?

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.

Key Findings

  • 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.


  • U.S. intervention decisions can be improved to better advance U.S. interests if contextual factors are carefully weighed and analyzed. This report reduces the complexity of this decision for senior leaders by identifying the key factors that should be weighed when making this decision and assessing the likely implications of specific intervention decisions executed in particular contexts.
  • Policymakers should consider the following four criteria when deciding whether a military intervention is likely to advance U.S. interests or prove counterproductive or wasteful: (1) how the intervention is likely to affect the local balance of power, (2) the role of external actors acting as spoilers, (3) the potential risks of escalation of the conflict or crisis, and (4) the degree of international legitimacy and the intervention's effect on U.S. reputation.
  • Earlier, larger interventions are higher risk and higher reward. These interventions are overall more likely to result in positive outcomes for the United States. However, earlier and larger interventions are also associated with some of the most-negative outcomes observed and, even when successful, involve substantial economic costs for the United States. Policymakers should carefully consider the likelihood of success and risks of failure before initiating such an intervention.
  • Intelligence collection to inform intervention decisions should focus on the following areas: the relative strength of the warring parties, the motivations of the parties, the identity of potential external spoilers and influencers, the prospects for support or backlash from local populations, likely adversary responses, and the likely response of the international community.

Research conducted by

The research described in this report was sponsored by the United States Army and conducted by the Strategy, Doctrine, and Resources Program within RAND Arroyo Center.

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