Go back to 1911 and convince Taft to concede the Republican nomination to Roosevelt. That allows Roosevelt to stomp Wilson, get the US into the war before Russia left, and get the war over with years earlier.

This prevents both Stalin and Hitler from rising to power, and prevents most of the European theater of WW2, as well as a host of other knock-on effects.

https://youtu.be/hLiI6kXZkZI?si=YJZMmkpOH4FZQiLt