The United States has a history of conducting large, conventional, firepower centric wars to achieve victory. This tactic hindered the U.S. approach to counterinsurgency (COIN) since Vietnam. The U.S. consistently failed to recognize it was fighting an insurgency and instead tried to fight the American Way of War. The inability of the military to recognize an insurgency and provide a consolidated, comprehensive, and coherent COIN strategy began in the 1960s and continued through today. This inability has proven to be a significant failure for U.S. strategic interests around the world. From Vietnam until today, the U.S. has failed to learn the essential lesson: large, conventional units cannot do nation building or COIN operations due to their size, their inability to conduct de-centralized operations, and their reliance on heavy firepower. This book examines how U.S. COIN doctrine evolved since Vietnam through a review of historical COIN trends from which U.S. COIN strategy was developed before Vietnam, revised during the interwar years, and reinvented for Iraq, and Afghanistan. This book demonstrates that U.S. doctrine does not show how to link the tactical to strategic applications of COIN properly for success. It then makes recommendations for the future of U.S. counterinsurgency operations.