In the culminating achievements of a lengthy career in the U.S. Army, George W. Goethals led the construction of the Panama Canal and managed the effort to sustain an army of approximately four million soldiers during the final year of the First World War. At the outset of that career, neither he nor the U.S. Army was prepared to meet such challenges. Using biography as a vehicle to examine a larger problem, this dissertation follows Goethals’s career in order to understand the nature of change in the U.S. Army during the late-nineteenth and early-twentieth centuries as it transitioned from a frontier constabulary and coastal defense force to a modern army capable of projecting power abroad and meeting the challenges of twentieth-century warfare. Goethals’s story reveals that the legacy of the Civil War and the army’s failure to keep pace with changing social norms and practices in training, education, and perceptions of professionalism created a traditionalist culture that did not embrace the new structures imposed by the Root Reforms at the turn of the century, but sought to apply them to comfortably traditional norms, values, and practices. At the same time, the army’s embrace of the managerial revolution and its experience of the First World War provided the impetus and momentum for cultural change. Reform was actually a decades-long process that was not complete until the army’s culture shifted to realign with its new structures.