The scale of the First World War, and in particular the high numbers of killed and wounded, marked the conflict as one of the most significant events of the twentieth century. For the first time, psychiatric casualties were not only a medical priority but also presented as a military crisis. In a protracted war of attrition, shell shock had the capacity to erode morale and undermine the fighting strength of the major combatants. Some senior physicians, such as Gordon Holmes, interpreted shell shock in the absence of a head wound as little more than cowardice, whilst others, including Charles Myers and Frederick Mott, explored ideas of psychological vulnerability and sought to correlate its symptoms with traumatic exposure. Clinical presentations differed between armies. In the UK, shell shock was commonly represented as a movement disorder, characterised by tremor and unusual gaits. This stood in contrast to Germany and Italy where seizures and dissociated, soldier-like actions were more commonly reported. Possible explanations for these national differences will be discussed in the context of combat medical services.