The previously unstudied records of the Japanese Red Cross Society (JRCS) narrate the heroic tale of the Red Cross personnel who carried out the organisation’s emergency response in the immediate aftermath of the nuclear detonations in Hiroshima and Nagasaki in August 1945. Aid workers began relief activities on the day of the bombings, while many themselves were suffering from injuries and radiation sickness. They treated civilians, soldiers, and Allied POWs without discrimination. Why did these aid-givers choose to work for the Red Cross in the face of such extreme risk? Why were they willing to walk into ground zero to render aid to survivors? JRCS records make clear that, as well-trained nurses and doctors, most aid workers understood the mortal dangers they faced from radioactive poisoning. This article argues that their actions in Hiroshima and Nagasaki were attributable to strict quasi-military training, which transcended the ideology of the JRCS as an imperialist institution. Their emergency responses demonstrate the extent to which Red Cross workers had internalised their professional mission as medical aid-givers and eventually emerged as exemplars of the humanitarian relief ideal.