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• The closer the relationship between the target country of the intervention and the United States and the greater the severity of the conflict, crisis, or threat to which the United States is responding, the greater the likelihood of a U.S. ground intervention.
• The United States has been substantially more likely to intervene in a country when it is has intervened in that country within the past 10 years, and has been more likely to conduct stability operations in a country when it was involved in a preceding combat phase.
• Interventions into armed conflicts have been less likely following large numbers of U.S. combat deaths — evidence of "war weariness" — but this effect does not extend to other types of interventions.
• Interventions have clustered geographically; the United States has been more likely to intervene in a country if it has recently intervened in a nearby country (within 1,000 kilometers).
• The number of wars going on in the world and the U.S. domestic political context have not appeared to affect the likelihood or size of an intervention.
• While deterrence interventions have tended to be larger in cases involving close U.S. allies, interventions into armed conflicts and stability operations in close allies have tended to be smaller.
• The actual duration of an intervention has tended to be significantly longer than planned for or expected.

In recent years, the frequency of U.S. military interventions in overseas areas, including not only those involving conventional war but also peacekeeping and humanitarian relief operations, has risen. These interventions have involved thousands of troops, cost billions of dollars, and placed significant demands on Army leadership, planning, and resources. The Army would benefit from an enhanced ability to anticipate the types and conditions of overseas military interventions it is most likely to be called upon to undertake in the future. This report constructs three different sets of models using historical data (one for each of three intervention types: interventions into armed conflict, stability operations in conflict and postconflict environments, and deterrent interventions). It examines the key factors influencing the incidence of military interventions and intervention size. Finally, the analysis provides the Army with signposts and metrics that can be used to identify countries, conflicts, and crises that are at highest risk for a U.S. intervention. Key signposts include the relationship between the target of the intervention and the United States, past U.S. military involvement in that country, and the severity of the crisis or threat to which the United States is responding. These signposts would allow the Army to better anticipate and plan for future interventions and could improve both near- and medium-term force-planning decisions.

• Chapter One

  Introduction

• Chapter Two

  Historical Analysis of U.S. Army Interventions

• Chapter Three

  Determinants of the Likelihood and Size of Interventions

• Chapter Four

  Clustering

• Chapter Five

  Case Studies

• Chapter Six

  Signposts of Future Interventions

• Chapter Seven

  Implications for Army Planners

• Appendix A

  RUGID Codebook and Methodology

• Appendix B

  Case Studies

• Appendix C

  Trends in Unit Type Use

• Appendix D

  Statistical Appendix