This article examines the institutionalization process of military hygiene expertise in Japan during the second half of the nineteenth century. It presents the principles, actors, institutional mechanisms and reforms that have contributed to the establishment of a modern health service. Immersed as they were in a cultural and political context of Western interference and civil wars, but also in the context of the generalization of conscription since 1873, doctors were assigned a new role: the preservation of the masses of soldiers' health. Along with this recognition of the profession, the Japanese army has integrated a growing number of sanitary auxiliaries, nurses and philanthropic associations as part of its development strategy. The analysis of the professionalization process of the military health service highlights the co-construction of medical expertise and of the modern state system as two facets of the same effort to mobilize and restructure the social body.