Both traumatic deployment experiences and antecedent traumas increase personnel's risk of developing PTSD and depression. However, only cross-sectional studies have assessed whether antecedent trauma moderates stress reactions to deployment experiences. This study prospectively examines whether antecedent trauma moderates the association between deployment trauma and post-deployment PTSD and depressive symptoms after accounting for antecedent mental health problems, in a large Australian Defence Force (ADF) sample.In the ADF Middle East Area of Operations Prospective Study, currently-serving military personnel deployed to Afghanistan across 2010-2012 (n = 1122) completed self-reported measures at pre-deployment and post-deployment.Within multivariable regressions, associations between deployment trauma and PTSD and depressive symptoms at post-deployment were stronger for personnel with greater antecedent trauma. However, once adjusting for antecedent mental health problems, these significant interaction effects disappeared. Instead, deployment-related trauma and antecedent mental health problems showed direct associations with post-deployment mental health problems. Antecedent trauma was also indirectly associated with post-deployment mental health problems through antecedent mental health problems. Similar associations were seen with prior combat exposure as a moderator.Antecedent and deployment trauma were reported retrospectively. Self-reports may also suffer from social desirability bias, especially at pre-deployment.Our main effects results support the pervasive and cumulative negative effect of trauma on military personnel, regardless of its source. While antecedent trauma does not amplify personnel's psychological response to deployment trauma, it is indirectly associated with increased post-deployment mental health problems. Antecedent mental health should be considered within pre-deployment prevention programs, and deployment-trauma within post-operational screening.