This Article addresses two interrelated questions. First, under the crime of aggression, applied retrospectively, who is liable for starting the First World War? Second, does the law's ascription of responsibility in this regard comport with our contemporary assessment of moral and historical responsibility for that armed conflict? The answer to the first question will satisfy many (Germany and Austria-Hungary), because it largely comports with what we have all been taught since grammar school. The answer to the second question, however, is counterintuitive to the point of consternation. Prevailing scholarly judgment among contemporary historians is that the leaders of several states were responsible for starting the First World War. Can international criminal law accommodate this empirical fact, i.e., can we interpret the law to fit the causal realities of such wars? If not, then there is only limited overlap between the class of those historically and morally responsible for events like the First World War and that class of those potentially liable for the crime of aggression, as currently defined and interpreted. This is no mere antiquarian curiosity, but has considerable relevance to a number of highly complex, multi-party wars, especially those in Africa's Great Lakes region extending over the last generation. The Article proposes several practical solutions to the problem, which we may understand as a challenge involving the simultaneous over- and under-inclusiveness of the crime of aggression in relation to the reality of causally complicated wars.