When the U.S. military invaded Iraq, it lacked a common understanding of the problems inherent in counterinsurgency campaigns. It had neither studied them, nor developed doctrine and tactics to deal with them. It is fair to say that in 2003, most Army officers knew more about the U.S. Civil War than about counterinsurgency. This volume was written to fill that void. The result of unprecedented collaboration among top U.S. military experts, scholars, and practitioners in the field, the manual espouses an approach to combat that emphasizes constant adaptation and learning, the importance of decentralized decision making, the need to understand local politics and customs, and the key role of intelligence in winning the support of the population. The manual also emphasizes the paradoxical and often counterintuitive nature of counterinsurgency operations: sometimes the more you protect your forces, the less secure you are; sometimes the more force you use, the less effective it is; sometimes doing nothing is the best reaction. A new introduction by Sarah Sewall, director of the Carr Center for Human Rights Policy at Harvard’s Kennedy School of Government, places the manual in critical and historical perspective, explaining the significance and potential impact of this revolutionary challenge to conventional U.S. military doctrine. An attempt by our military to redefine itself in the aftermath of 9/11 and the new world of international terrorism, The U.S. Army/Marine Corps Counterinsurgency Field Manual will play a vital role in American military campaigns for years to come.