WWII vets had a different view of life and death. They had lived very different lives than people who went to Vietnam and later wars. Chances were very good that a WWII vet had lost a sibling or childhood friend to polio, smallpox, measles, diphtheria, or strep throat, or a blood infection from a cut or blister. A much greater number of them had lost a parent at a young age. A much greater number of the had hunted for sustenance, and had participated in the slaughter of farm animals, or had shopped at butcher shops, where they had to order cuts in such a way that it was clear they were asking the butcher to cut a hunk off a once-living animal. They bought chickens and geese with the heads still attached.
The world that WWII veterans came from was a lot harsher than the one that their children and grandchildren who went to Vietnam faced, and very, very far-removed from the world of Gulf War and Iraq soldiers.
Another consideration is that the average of a WWII recruit was about 27, while a Vietnam recruit was about 19. That made a huge difference between those to generations.
In the Gulf and Iraq, we were back to an average age of about 25-6, but then, we had people who had had really sheltered lives by that point, compared to the conditions in war, plus weapons of terrible destruction, and from Vietnam through Iraq, more of a blur between military and civilian targets.
My husband was in Iraq. The truck drivers he worked with were told to keep driving even if there was a child in the road, because insurgents would deliberately place a child in the road to stop drivers, and then shoot them, or shell the truck. WWII vets didn’t face things like that.
Also, WWII vets came home to ticker tape parades. Vietnam vets got spit on. Gulf and Iraq vets came home to people saying “Thanks for serving,” but to unemployment, for as long as they remained in the reserves. My husband got turned down for job after job, and limped along on Manpower, and a part-time security job. As soon as he was discharged from the reserves, he got the next job he applied for.
Everything I said applies to WWI and Civil War vets as well.
And yet, there still was “shell shock,” and “battle fatigue.” So it was not as though there was NO PTSD, but I think there were reasons that complicated and multiplied it in later wars.