There was shell-shock... But aside from that extreme diagnosis? Most people were simply "changed by the war", "came home a changed man", "never got over the loss of her sons", "got all kinds of 'ideas' into her head after driving ambulance", "after the great war, she became very wild. She was a nurse, you know. She wasn't always this way," etc. And, at least on the Allies side? It was nearly 2 entire generations of young men. Men were actually shooting themselves / committing suicide if they were declared 4F / non-deployable and couldn't enlist.
After WWI, while Germany was plunged into starvation and a Great Depression?... We had the Roaring 20s. Jazz; The original PTSD music playlist. (Anything goes!). Sex-rules changed almost overnight. Crime skyrocketed. Alcohol use skyrocketed. The art scene exploded. Even Paris, with the Starving Winter? Might have been starving, but the parties raged on. Nearly the whole decade looked like one big fat Military-Unit-On-Leave. Conversely... Record numbers took to the wilderness, backcountry, bush, outback, plains. Either as hermits or gold/ copper/diamond & other mineral rushers. Lawless mining camps. Back of Beyond with only the rules you make. Meanwhile others pulled up stake and headed over to China's rebellion, or Russia's. Swept into either the ideals of communism, or another fight, or both.
Do I think it was all PTSD? Nope. But if we take modern numbers, at apx 20%? Inflate another 10% for years fighting without a break, to arrive at still a very conservative 30%? That's 1/3 of millions. And it very easily explains nearly a decade of living as if it were the last day of your life. The whole damn decade looks like 1 super clichéd PTSD Tailspin. Who knows how long this would have gone on for in the States, if the stock market hadn't crashed in the 30's? Suffered our own Great Depression?
In the States... We're taught that we created Hitler at Versailles. Forced both him and his rule into being during the Peace Accords. Our children shoulder that blame. I've gone to dozens of schools, and while the curriculum often changed depending on what state I was in? That was one concept that was true in every school I attended; WWII was our fault. We created Hitler. This, this is what happens when you take your revenge on a national scale. We also tend to shoulder a big damn chunk of the blame of the Halocaust; we turned boats of refugees away. Sent hundreds of thousands to their deaths. Worse, we sat on the sidelines and did nothing. For years. Even as Austria and others were being invaded. Even during the London Blitz. Even as report after report after report came in? We just wanted to be left alone. Leave us alone. Go away. f*ck the world. The actual "name" for this policy was "Isolationism". Still sounds a little PTSD-ish, doesn't it?
2 more generations of American men, one young, one not so young (vets of WWI) finally shipped off. And returned. Several years later.
Using the roughly 1/3 rule, again, and accounting for some overlap? Let's say roughly 40%-50% of the men in America would have had diagnosable (by today's standards). Women, too. But I don't have solid numbers of female combatants & medics & resistance & prison camp survivors & rape victims & refugees. So, just know that by leaving us out, these numbers are all nice and extra conservative.
For the Continent... It would be worse/higher. It wasn't just the adults fighting, but a couple generations of kids also picking up combatPTSD, living though 1 war, fighting through another. Or born into it.
You want to know Marriages in older Vets with PTSD?
Easy.
Roughly half of our Grandparents (if you're my age, great grandparents or parents if you're younger or older), one or both partners would likely have had combatPTSD. That's a couple hundred thousand marriages to pull data from at least. Probably several million. I'd have to look up population stats for 1900-1950 across a dozen countries to be more accurate. A whole lot, though, roughly half the population.