They are wrong.
We could have done better with our post-war settlements, but Hitler and WWII could not have been “precluded.”
These are the same dingbats that can argue that if Shakespeare were alive today he’d be writing sitcoms. There’s no proving or disproving such stupid arguments. All you can do is point to the facts.
WWI stopped tyrannical aggression and helped unite freedom-loving countries in such a way that they would form an alliance that would survive the rest of the century and thus preserve democracy.
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.