Heck, if humanity turned out to have a real problem with nuclear weapons--like people simply treated them like normal weapons, the United State's destruction would appear inevitable, and other nations would learn to build deep underground, to safety. The United States, relying so heavily on oceans for its defense, failed to adapt to a model of war where "everyone that isn't 120' underground dies", so they're as dead as a dinosaur.
With a PoD of 1961, you could therefore add in a third and fourth world war to entirely beat the hell out of the United States and knock them out of ever being able to claim membership to a first world nation. Just keep glassing American cities and they'll stop getting built.