The diagnosis, Posttraumatic Stress Disorder, was introduced in 1980 amidst debate about the psychiatric toll of the Vietnam War. There is controversy, however, about its central assumption that potentially traumatic stressors are more important than personal vulnerability in causing the disorder. We tested this assumption with data from a rigorously diagnosed male subsample (n = 260) from the National Vietnam Veterans Readjustment Study. Combat exposure, pre-war vulnerability, and involvement in harming civilians or prisoners were examined, with only combat exposure proving necessary for disorder onset. While none of the three factors proved sufficient, estimated onset reached 97% for veterans high on all three, with harm to civilians or prisoners showing the largest independent contribution. Severity of combat exposure proved more important than pre-war vulnerability in onset; pre-war vulnerability at least as important in long-term persistence. Implications for the primacy of the stressor assumption, further research, and policy are discussed.