Have there be no recognizable Second World War. There still has to be a first one sometime in the early 1900s, but there cannot be a second. Bonus points if WW1 still happens as OTL.
I can't decide if that's better or worse than this National Lampoon VW ad.A time traveling Mercedes runs over a young Adolf Hitler.
The economic turmoil resulting from the Great War will ensure the rise of socialism in various forms.