In an attempt to understand the micro-dimensional mechanisms of how some individuals come to perpetrate violence against civilians during wartime, this thesis adopts a theory from organisational psychology. By looking at the military as a professional organisation, violence against civilians perpetrated by state armies during wartime is theorised to be the outcome of a process of script error wherein military scripts of non-combatant immunity fail. The theory is applied on the massacre in My Lai, during the Vietnam war. Findings showed that the mechanism of script error did not play out completely as theorised, however that military scripts did dictate behaviour and that a script error was present to some degree as civilians came to be targeted as if they were enemies. Some mechanisms used in previous research on violence against civilians were supported by this study and could also be integrated into the framework of organisational scripts, showing the explanatory value that organisational scripts have to further understand military violence. The study contributes to a deeper understanding of an important historical case, shows the value of introducing organisational psychology into studies of the military organisation and finally helps us further make sense of situations of violent transgression.  organisational scripts, script error, military violence, violence against civilians, mass violence, atrocity, My Lai