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 they did about counterinsurgency. The U.S. Army / Marine Corps Counterinsurgency Field Manual 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;...