What's the worst that could happen? I foresee only these things in the near term:
China moves on Taiwan, which they are bound to do sooner or later anyway. They may then look at future military expansion in Central Asia.
Iran and Israel fight a war that may or may not go nuclear.
Afghanistan falls once again to Medieval brutes, which is bound to happen sooner or later anyway. Iraq breaks up into Kurdistan and two satellite states of Iran and Syria.
Russia reclaims some of the former Soviet Republics/Tsarist imperial lands. They're sure to dominate the Caucuses and maybe move against the Baltic Republics, which is bound to happen sooner or later anyway. If Russia invaded Latvia, Lithuania, and Estonia tomorrow, would the US and NATO really have an earth destroying WWIII over these tiny countries that barely existed before 1990?
Communism falls out of favor over time in Latin America. Cuba, Venezuela, and all the other Marxist/radical left movements no longer have the American boogeyman to build populist support on.
Japan gets over its postwar military phobia and, with South Korea, is more than capable of defending the relatively free parts of Asia (that aren't claimed by China). I wouldn't be averse to leaving S. Kore and Japan a couple nukes before we pull our troops out.
Finally, Islamic terrorists are put out of business or soon find targets other than America to be concerned with. Neo-con fantasies about them "following us home for revenge" are just that: fantasies.
