This essay explores one of the most prominent, although seldom acknowledged, use cases of design: military technology. We explore the ethical and practical challenges by using three cases where HCI, the military and war-fighting have become interdependent. First, the development of head mounted displays raises the problem of the lion and the lamb', where it is near impossible to separate military and civilian applications. Second, we look at how maker-spaces have come to be used as a way of engaging civilians in building and fabricating military hardware. Lastly, we look at the adoption of AI and robots, and how robots and drones are developing into a new class of weapon. In discussion, we consider how HCI could critically reflect upon the application of the technologies it helps to design, and opportunities for a harm reduction' human computer interaction.