This article traces the history of the British National Society for Aid to the Sick and Wounded in War (NAS), and its interventions in Continental and colonial wars of the late-nineteenth century. The NAS was founded on the outbreak of the Franco-Prussian War in August 1870. It went on to become one of the most important founding members of the British Red Cross Society (BRCS) when it was established in 1905. The aim of the article is to uncover the particular anxieties and aspirations that contributed to the foundation of the NAS. It demonstrates how these concerns -many of them related to the relative state of the British military- informed its subsequent practices and its relationship with the International Committee of the Red Cross. In tracing its emergence as a paramilitary corps adept at rapid-response emergency medicine, this article uncovers the rivalry that characterized attempts within the NAS and BRCS to lay claim to the "true spirit" of voluntary aid in war -a rivalry which eventually informed British insistence on a revision to the Geneva Convention in 1906.