It's really simple
After WW2 the US decides to adopt an isolationist policy.
No more involvement in Europe or Asian or even South American affairs.
Using my imagination and seeding it with actual historic events I can see.
Communist Russia taking over all of Europe.
Communist China taking over all of Asia.
Communist Cuba taking over all of South America.
Communist clients taking over all of the Middle-East.
Eventually there would be a Communist takeover in Canada and the United States.
Life would not be as it is now.
Perhaps you have a better vision.