5. The West is dying. Literally - our creation of an entitlement culture and our devotion to materialism have left us with birthrates below replacement level. In both Europe and America the solution has been mass immigration - but both have had issues with assimilation. America here is comparatively lucky, her immigrants share many of her cultural assumptions. But Europe is not - the West in Europe is being replaced by a high-birthrate Islamic culture which does not accept the Enlightenment. As the immigrant populations threaten to break the local safety nets and culture, the backlash they provoke isn't what we would recognize as classic liberalism, but rather classic fascism. Nationalist groups are springing up all over Europe, though they are doomed by their own inability to breed to dying out after sparking conflict. All those aspects of the West that we consider dear; the rights of the individual, limited, secular government, free markets... they are doomed to wither and die as the culture that upholds them does.
The situation at current cannot sustain indefinitely - eventually the destabilizing elements that are currently inherent in the Middle East will win, and the price of loss is not just a world wide economic collapse, but the slide, decline, and perhaps fall of the West. The long-term solution is therefore to change the rules of the game. The destabilizing elements in the Middle East must be replaced with stabilizing ones. Tyrannies must (carefully) be replaced with representative governments that give public pressure an outlet other than violent overthrow. Rentier societies that encourage stagnation, revolution, and hostility abroad must be replaced with market economies that encourage trade, growth, and a politically active middle class with a vested interest in stability. Radical Islam must be replaced with a new ideology that allows Muslims to recoup their pride and independence without striking at others. In short, we need to allow the Enlightenment to do to Islam what it has done to Christianity.
Even with our presence, US pursual of that strategy (again, as we see today) is not guaranteed, and even with US pursual of that strategy, sucess is not any kind of certain.... but if the US withdraws before these things are accomplished (or, at least, accomplished enough to become self-feeding cycles), then the game is up. the match is struck. Europe falls, China moves to become hegemon, nukes possibly fly, and back to the Dark Ages we go, but this time with much, much better weapons with which to massacre each other in the name of God.
In the short run, a world without a global US presence is merely one in which Iran, India, China, Russia, Saudi Arabia, and Egypt all compete against each other for control over the worlds' critical choke points, turning our current global system into a series of mercantilist blocs, and causing economic meltdown here in the United States. In the long run, a world without a global US presence is a much, much, much uglier place. But hey, if you have evidence that the CCP or the Muslim Brotherhood leadership isn't a bunch of Nationalist Corporatist/Islamists willing to sacrifice nigh on anything to retain power, and is in fact a bunch of misunderstood liberal, free-traders who stay up late at nights reading Locke, I'd be glad to see it.