The extent to which combat exposure exacerbates the risk and persistence of PTSD is analysed in this chapter and related to the combat experiences of the World War I veteran in Chap. 1 and his emerging PTSD symptoms diagnosed in Chap. 2. The types and intensity of wartime stressors have been quantified and shown to correlate with greater current and lifetime prevalence of PTSD. Of these stressors witnessing death or injury, personal threats and killing are the strongest predictors of PTSD onset, with the extent of violent combat and proximity to death and explosive devices also prominent. Finally, it is shown that among the PTSD symptom clusters, it is re-experiencing intrusions that is most closely associated with violent combat and injury, as well as death and proximity to explosive devices.