Using an original data set of 145 ground, air, and naval interventions from 1898 through 2016, this report identifies those factors that have made U.S. military interventions more or less successful at achieving their political objectives. The factors that influence success vary depending on the type of objective being pursued, highlighting the need for careful analysis and resourcing decisions before policymakers initiate military interventions.
Characteristics of Successful U.S. Military Interventions
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- What types of political objectives has the United States historically pursued through military interventions?
- How successful has the United States been historically at achieving political objectives during military operations?
- What are the characteristics of U.S. military interventions that are most likely to achieve their political objectives?
- How do such factors as the size of the intervention, the operating context, the local dynamics, and the international system influence the outcome of the intervention?
Using an original data set of 145 ground, air, and naval interventions from 1898 through 2016, this report identifies those factors that have made U.S. military interventions more or less successful at achieving their political objectives. While these objectives were often successfully achieved, about 63 percent of the time overall, levels of success have been declining over time as the United States has pursued increasingly ambitious objectives.
The research combines statistical analysis and detailed case studies of three types of interventions — combat, stability operations, and deterrence. The research highlights that the factors that promote the successful achievement of political objectives vary by the nature of the objective and the intervention. For example, sending additional ground forces may help to defeat adversaries in combat missions but may have a more contingent effect on success in institution-building in stability operations, where nonmilitary resources and pre-intervention planning may be especially vital.
The report offers five main policy recommendations. First, planners should carefully match political objectives to strategy because factors that promote success vary substantially by objective type. Second, sending more forces does not always promote success, but for certain types of objectives and interventions, greater capabilities may be essential. Third, policymakers should have realistic expectations regarding the possibility of achieving highly ambitious objectives. Fourth, pre-intervention planning is crucial. Finally, policymakers should carefully evaluate the role that might be played by third parties, which is often underappreciated.
- U.S. political objectives in military interventions were often successfully achieved, about 63 percent of the time, with clear failure to achieve them relatively rare, about 8 percent of the time.
- U.S. objectives have tended to become more ambitious over time, and this shift has corresponded with a gradually decreasing likelihood that objectives will be successfully achieved.
Combat and counterinsurgency interventions
- Particularly in the post-1945 era, the United States has generally been able to achieve its objectives in these interventions when it applies substantial numbers of forces, and particularly ground forces.
- The effectiveness of U.S. forces in achieving objectives in combat interventions may be augmented by the often-superior technical capabilities of the U.S. military.
- Pre-intervention planning is a key factor influencing the ability of the United States to achieve its objectives.
- The ability of the United States to focus on and achieve its political objectives in stability operations appears to diminish as the intensity of conflict increases.
- Nonmilitary resources and pre-intervention planning can be central to success.
- The initial quality of host-nation political institutions and the support of the host-nation government can have a substantial effect on the success of stability operations.
- Third-party interference can substantially affect the likelihood of success.
- The United States appears to have been more successful in its pursuit of its objectives in deterrence interventions at points when the country's military capabilities vis-à-vis the rest of the world were higher, as was the case immediately after World War II.
- Match intervention strategy with objectives. The factors that promote success often vary, depending on the type of objective being pursued. Before intervening, the United States should consider the political objectives it hopes to achieve and then consider the mix of military and nonmilitary tools best suited to achieving those objectives.
- Ensure sufficient force size. Sending more forces does not always promote success, but for certain types of objectives and interventions, greater capabilities may be essential. Military planners should consider not only the operating context but also the specific political objectives to best determine the appropriate number of forces.
- Be realistic about expectations for success. Highly ambitious objectives cannot necessarily be achieved through the application of larger military forces. Broadly defined goals may also require diplomatic or other contributions, which may be difficult to coordinate and integrate into military missions or may simply not be available.
- Pre-intervention planning should be comprehensive. The United States military has historically prepared well for combat interventions but has focused less on planning for the postconflict stabilization phase. The same rigorous and detailed planning that goes into preparing for and scripting combat operations would benefit efforts involving institution-building and peacekeeping.
- Evaluate the possible role played by third-party interference. The role played by third parties appears underappreciated in numerous past interventions, and this can have deleterious consequences, particularly for deterrence and stability operations. The presence of third-party actors may increase the number of forces required or highlight the importance of a regional diplomatic strategy for success.
Table of Contents
Existing Literature: Findings and Shortcomings
Collecting Data on U.S. Military Interventions
Factors That Affect Success: A Quantitative Look
Factors That Affect Success: Case Studies
Synthesis and Recommendations
Codebook: Data Collection and Definitions
List of Cases