U.S. armed forces are commonly described as inflexible, too easily caught out by the unexpected features of a conflict. Barno and Bensahel trace the factors that enable more nimble adaptability: technology, shrewd leadership, and sound doctrine. They examine how these factors have helped account for U.S. military successes and failures in operations dating back to World War II. U.S. armed forces adapted well in learning to work with the Northern Alliance in Afghanistan in 2001 and with tribal coalitions during the so-called Anbar Awakening in Iraq in 2005. Then, however, General David McKiernan in Afghanistan and General George Casey in Iraq struggled to adjust their tactics to conditions on the ground and so failed to stem the tide of violence. The authors warn of the coming radical changes in the strategic environment, including increasing tension with China, and argue that the U.S. military has to transform the way it goes about its business, resisting doctrinal rigidity and organizational inertia and finding new kinds of leaders. This is a thoughtful and informed analysis, even though it has quite a narrow focus on land forces and doesn’t consider the political contexts of U.S. military operations, especially in conflicts in which the United States has had to work closely with local forces.