How can post traumatic stress disorder effect war veterans?

Harshly: Veterens with post-traumatic stress disorder ("ptsd") often have intrusive memories of an experience in which they felt extreme threat to their physical or psychological well-being. These intrusive memories can be in the form of flashbacks, nightmares, or even "re-living" the trauma in the present. Other symptoms can include avoiding places that remind of the trauma or just feeling numb.

PTSD: Dr berman's answer hit all the big points. The majority of combat vets don't develop ptsd. Some vets have mild sxs. But many have moderate to severe sxs that leave them feeling unsafe with difficulty sleeping at night. They may be over-reactive & quickly go into fight or flight mode. They avoid (or sometimes are drawn to) people, places or things that trigger nightmares, memories & flashbacks.

Threat of death: PTSD is a post trauma reaction to being in the situation of impending real or perceived loss of life, or inability to is scape. Examples of this would come from being in combat situations, or severe physical or sexual abuse. There are degrees of PTSD, it is not a matter of either the patient does or does not have it, it's not all or none.