This article examines post-traumatic stress disorder (PTSD) among personnel of the Nigerian Army (NA) and its implications for national security between 1999 and 2020. The article argues that PTSD should not be treated solely as an individual medical problem because the mental health of military personnel is connected to operational effectiveness, family stability, troop availability, discipline, and the wider capacity of the state to provide security. The study employed historical, narrative, and analytical approaches and drew on primary and secondary materials. The research identifies prolonged deployment, combat exposure, death and injury of colleagues, captivity, sleep deprivation, fatigue, fear, violent encounters, and other operational experiences as important sources of trauma. It also identifies stigma, inadequate mental health personnel and facilities, weak data systems, low awareness, fragmented management arrangements, and insufficient collaboration as barriers to effective PTSD management. The article further examines the effects of PTSD on the protection of lives and property, troop availability, citizen well-being, family relationships, and socio-economic activity. Drawing on systems theory, it demonstrates how difficulties within the individual military subsystem can spread through the family, organizational, and societal systems and eventually affect national security. The article concludes that effective PTSD management requires a coordinated mental health policy, improved specialist capacity, and stronger inter-service and civilian collaboration.