This Field Manual/Marine Corps warfighting publication, "Counterinsurgency," establishes doctrine (fundamental principles) for military operations in a counterinsurgency (COIN) environment. It is based on lessons learned from previous counterinsurgencies and contemporary operations. It is also based on existing interim doctrine and doctrine recently developed. Counterinsurgency operations generally have been neglected in broader American military doctrine and national security policies since the end of the Vietnam War over 30 years ago. This manual is designed to reverse that trend. It is also designed to merge traditional approaches to COIN with the realities of a new international arena shaped by technological advances, globalization, and the spread of extremist ideologies-some of them claiming the authority of a religious faith. The manual begins with a description of insurgencies and counterinsurgencies. The first chapter includes a set of principles and imperatives necessary for successful COIN operations. Chapter 2 discusses nonmilitary organizations commonly involved in COIN operations and principles for integrating military and civilian activities. Chapter 3 addresses aspects of intelligence specific to COIN operations. The next two chapters discuss the design and execution of those operations. Developing host-nation security forces, an essential aspect of successful COIN operations, is the subject of chapter 6. Leadership and ethical concerns are addressed in chapter 7. Chapter 8, which concerns sustainment of COIN operations, concludes the basic manual. The appendixes contain useful supplemental information. Appendix A discusses factors to consider during the planning, preparation, execution, and assessment of a COIN operation. Appendixes B and C contain supplemental intelligence information. Appendix D addresses legal concerns. Appendix E describes the role of airpower.