This dissertation explores U.S. military surgeons’ purposeful efforts to alter how medical and line officers in the U.S. Army and Navy conceived of disease, appreciated surgeons’ roles, and organized medical war preparations through education, training, exposure, and medico-military professionalization between 1884 and 1918. It traces surgeons’ postwar efforts to change American military cultures in response to the revelations of the germ theory of disease and deadly typhoid fever epidemics in the American training camps of the Spanish-American War. Medical and line officers required academic education and practical lessons to contextualize disease, surgeons, and medical care, understand and appreciate germs’ role in medicine, and train to apply these lessons to benefit their soldiers and sailors. Surgeons also reinforced their scientific education and grew military medicine through postgraduate education and tactical training designed to enhance the line’s perception of surgeons and medical science.This dissertation rests on the contention that surgeons contributed to military preparation for the next war by effecting cultural change to prevent the epidemics of previous wars. This culture of medical preparation shaped how military medical departments recruited, organized, and trained medical officers, procured supplies, and managed civil-military relationships. Entwined cultural change and war preparation were expressed in the multiple mobilization activities through which surgeons validated the success or failure of their efforts. Troops participated in organized camps of instruction, maneuver camps, and major mobilizations to the U.S.-Mexico border, allowing surgeons to use the physical encampments, hospitals, and other surgeons to test assumptions, exercise and refine theory, validate operational principles, and improve from previous iterations. As the United States entered the Great War in 1917, epidemics of measles, influenza, and meningitis attacked Army and Navy recruit training camps. Rather than demonstrate failure, this dissertation positions the 1917 and 1918 epidemics to demonstrate medical officers’ successful military cultural change. A comparative approach between 1898 and 1918 also highlights cultural and medico-military evolution through the lenses of preparation and mobilization. Official military reports and archival sources illuminate cultural divisions between line and medical officers and track the curricular development of military hygiene and sanitation courses in undergraduate and professional military schools and specialized fields at military medical schools. This dissertation intervenes in military and medical historiographies by pushing the conversation beyond disease’s impact on war to center disease and changing perceptions of disease, culturally and medically, as features of military preparation. It also recasts military surgeons as central agents in the U.S. military’s turn-of-the-century professionalization and modernization efforts. As the world addresses the outcomes and aftermath of the COVID-19 pandemic, this dissertation demonstrates that physicians and societies met previous epidemics and pandemics on medical science’s past frontiers where the germ theory of disease had barely won acceptance. It also illustrates the power of individuals in subordinate classes to affect institutional cultures for the betterment of all. Lastly, as military operations during future pandemics are all but guaranteed, this dissertation proves that dedication and preparation are just as vital to epidemic defense as good science. / History