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Small Interventions Can Reduce the Odds of Defeat, but Not Even Large Interventions Can Significantly Improve the Chances of Outright Victory

• Foreign interventions can help prevent the defeat of a partner government, but they do not, on average, increase the chances of a decisive military victory.
• Interventions of at least 1,000 soldiers roughly double the probability of achieving a negotiated settlement between the government and rebels, and larger interventions can improve these odds still more — albeit at a diminishing rate of return.
• This inability of foreign military force to secure victory holds true regardless of the number of troops deployed.

Foreign Interventions by Non-Western Countries into Ongoing Conflicts Are Generally Associated with Higher Levels of Violence

• The United States, France, and United Kingdom, on average, neither worsen nor improve levels of violence during ongoing wars.
• Interventions by other countries are associated with more-intense conflicts.

Foreign Interventions Are Associated with Much Longer Wars

• Interventions are associated with wars lasting some 50 to 60 percent longer than those not experiencing intervention.

The Durability of the Outcomes Achieved Through Foreign Military Intervention Depends on the Level of Development of the Partner State and the Identity of the Intervening State

• Roughly half of wars, once ended, will resume again within a quarter century or less, and more than one-third relapse into violence within five years of the end of the earlier war.
• Less developed countries are much more likely to experience conflict recurrence in the wake of foreign intervention.
• Less developed countries are much more likely to experience conflict recurrence in the wake of foreign intervention.

Drone Strikes Have Had Opposite Effects Depending on the Intensity of the Drone Campaign and the Operational Environment in Which It Is Conducted

• The disruptive effects of drone strikes are limited to militant attacks.
• On average, these strikes do not appear to meaningfully disrupt and degrade militants' ability to produce propaganda.
• The effects of drone strikes decline rapidly over time, usually fading to statistical insignificance within six months.
• Where drone strikes are conducted only sporadically or in conjunction with ineffective local partners, they can be counterproductive.
• Successful leadership targeting has substantial effects — both favorable and unfavorable depending on the local context.

Indirect Options Are Limited in What They Can Accomplish on the Ground

• Efforts to contain conflicts by providing military assistance to neighboring partner states do not appear to be effective, at least within an operationally relevant time frame.
• Operations to mitigate the humanitarian consequences of wars — such as no-fly zones or safe areas — are typically much more costly and carry much greater risk than anticipated.
• Interdiction is considerably more promising as a supporting element in a strategy than as the central pillar of a strategy, let alone a policy used in isolation.

The foreign policy and defense communities have intensively debated the efficacy of low-cost and small-footprint military options for crises such as those in Afghanistan, Iraq, Libya, Somalia, Syria, and Yemen. This report divides these operations into three types: limited stabilization (involving the deployment of hundreds or thousands of ground forces to bring a conflict to a favorable end), limited strike (involving airpower — predominantly drone strikes — to disrupt and degrade militant networks), and indirect options to contain or mitigate a conflict. The report provides a statistical analysis of hundreds of cases, supplemented by case studies, to evaluate the strategic effects of each type of operation.

Limited stabilization missions can improve the odds of achieving an acceptable outcome at relatively low cost, but the odds of outright military victory are generally small. Larger numbers of forces, on average, yield better outcomes, but only at extremely high cost. The United States can instead rely on partners to conduct these operations, but doing so often comes with numerous drawbacks.

Limited strike operations can disrupt militant networks — but generally only when they are conducted intensively and in cooperation with a reasonably effective partner on the ground. Where these conditions do not hold, such strikes appear to have counterproductive effects, including increased militant attacks and propaganda activity.

Finally, indirect options were found to have limited effects. Efforts to bolster front-line states to prevent conflict spillover did not have any observable effect. Safe areas, no-fly zones, and interdiction campaigns can all provide important benefits — but usually as elements of a larger military operation, not as alternatives to large-scale intervention.

• Chapter One

  Introduction

• Chapter Two

  Limited Stabilization

• Chapter Three

  Limited Strike

• Chapter Four

  Indirect Options: Interdiction, Mitigation, and Containment

• Chapter Five

  Conclusions and Policy Implications

• Appendix A

  Models for Limited Stabilization

• Appendix B

  Limited Strike Statistical Models and Results

• Appendix C

  Statistical Models Assessing U.S. Containment Efforts

• Appendix D

  Case Studies for Mitigation

• Appendix E

  Case Studies for Interdiction

• Appendix F

  Costs of U.S. Military Interventions