How did American values change post-WWI?
Millions of American farm boys returned home with their previously-held values challenged to the breaking point.
The typical recruit/conscript for WWI was a rural farm boy who had never spent much time more than a mile or two from the place of his birth, tasted alcohol, kissed a girl or missed church on Sunday.
This same kid came back two or three years later. In addition to having seen horrors of war without precedent, he also saw Paris and/or London, took up smoking and whiskey, was exposed to modern art, European culture and communicable diseases, and retained little of the small town religion he'd been raised with.
He probably turned 21 in Europe and missed a couple opportunities to vote. In his absence, women got the vote for the first time (which was very different from what our soldier was used to) and they voted en masse to ban the booze (just as our soldier was learning to hold his liquor).
Tens of thousands of American military personnel opted to stay in Europe through the 1920s. The ones who went home retained a lot of the "Eat, drink and be merry, for tomorrow we die" ethos that had sustained them in wartime. The family farm they knew was different or gone, or anyway they didn't want to go back to it after having seen Paris. It kind of set the stage for the flappers, speakeasies and crazy stock trades of the following decade.