1) The mad mullahs of Iran gain a nuclear weapon and follow through on their threat to erase the Jewish state from the map. The Iranians see weakness in the West and foresee minimal retaliation. Perhaps they are wrong in this calculus; either way, seven million souls vanish from the planet and the West is complicit in another Holocaust.
2) Russia, seeing no push back from its invasion of Georgia and seeing the United States all but renounce its missile defenses, begins to swallow its former slave states — Ukraine, Poland, Hungary, et al — one by one. It shuts off fuel supplies to Western Europe and blackmails them into accepting this Soviet-light expansionism.
3) European society, no longer under any credible American protection and threatened from without by the encroaching Russia and from within by its increasingly violent immigrant population, crumbles. As has happened so often in the past, a dictator steps in to restore order.
4) Meanwhile, across the Atlantic, segments of the American population resist the sudden and convulsive socialization of their republic — peacefully first, then more forcefully. China refuses to buy any more of our debt, and trillion dollar deficits finally overwhelm the greatest economic engine the world has ever seen. The U.S. economy fails, dragging the entire world down into depression, sparking global violence and chaos. China moves on Taiwan and the North Koreans push south, both confident that the United States is in no position to stop them.