I think the most obvious failure arising from WWI was the League of Nations. To the left's credit, they were consistent enough to repeat their mistake by founding the UN after WWII.
As I said, we could have done better with the post-war settlement. The greedy Europeans split up some Muslim countries and added fuel to the already burning fire of militant Islam.
But at least our reps in both houses had the foresight to squash the LoN. It passed in only a shadow of Wilson’s intended tyranny over sovereign nations, the US included.