Of all the war veterans I've known, the majority of those who saw combat had a least a minimal degree of survivor's guilt. That is, the gratitude for being alive was challenged by the knowledge of friends who didn't return from the battlefield. The more morally attuned often report wondering "why did ____ die and why did I live? I wasn't any better as a person."
Lincoln addressed this, at least obliquely, in the Gettysburg Address, when he referred to "the brave men, living and dead" who had fought there, as a bridge to his exposition on why they fought. Gore Vidal puts in the mouth of one of his characters the speculation that Lincoln willed his own death at war's end, as payment for the awful suffering the war had inflicted on the nation. This may be a stretch, although Lincoln's moral sensitivity, sharpened by the loss of two of his own sons from illness, plus his lapses into melancholia, makes this seem not quite as preposterous as it sounds.
I knew of this phenomenon in the abstract, since I'm not a veteran. It became a reality to me in 1983. That year, my father's wartime unit went for a reunion at their wartime station, Grafton Underwood, England. One of the highlights of the reunion was the dedication of a memorial to the unit. This was done with appropriate pomp and solemnity, including salutes from U.S. forces stationed in the U.K. They engaged professionals to make a commemorative video. During the dedication ceremony, the camera panned across the ranks of the assembled veterans. On almost every face, there were tears evident. The waterworks had nothing to do with the English rains common at that time of year.
Memories of loss can cut with particular sharpness when they're fresh. When a friend or loved one dies, something as mundane as hearing that person's favorite song can make us cry. Over time, these memories lose their sharp edge and become part of what we remember with affection. They never completely lose their sting.
The story "Gold Stars" in Tangled Woods and Dark Waters is about the return of a young veteran from the European Theater in 1945 and how a visit to his boyhood church gives him the sense of being "a stranger in a strange land." It was originally written as part of a novel titled Kilroy's Shadow, which I've shelved as an active project. I'd be interested in hearing from any war veterans as to how it stacks up against their experiences on returning from a war.