How do traumatic experiences in individuals’ pasts impact political behaviour in the context of a new traumatic setting? I draw from established theories of how trauma can shape political participation and utilise the concepts of re-traumatization and risk-aversion to predict how battlefield outcomes vary due to past exposure to famine. I leverage individual-level data from the Union Army in the US Civil War to investigate how cohorts of Irish soldiers differ in their behaviour compared to counterfactual cohorts of soldiers from other immigrant groups. I validate this approach by observing the physical effects of famine on soldier heights and use various strategies such as estimating soldiers’ origins in Ireland to capture individual-level famine intensity. I find that soldiers born just before the potato famine participate at consistent rates but desert from the battlefield at higher rates than both their older compatriots or equivalent cohorts of other immigrants. I rule out an alternative economic mechanism and highlight that a risk-aversion mechanism best explains these elevated patterns of desertion. This research contributes to our understanding of the psycho-social effects of war and how behaviour during conflict is influenced by earlier trauma.