It would have a huge impact because the US is so involved all over the world, not to mention the US has provoked pretty much every non-western country in the world and we now face a war on terror BECAUSE of the US. Since the rest of the West supported the US when they rattled the snakes in the middle east and other places, we are now involved. If the US pulls out suddenly, it will be US history all over. The US left the Kurds to be gassed to death by Saddam Hussein when they first got them to fight and then pulled out in 1991. Hundreds of thousands of Kurds were killed because of the US.
If the US pulls out, islamist terrorists will still be upset with the West over something the US pulled us into. Also, Ron Paul wants to crash the economy. If the US falls, the world will be heavily affected. Also, Americans who are not middle class or rich will be screwed. In other words, we're all fekked. In addition, we'll see the Chinese and Russia gaining ground.
What the US needs to do is to secularise and make friends with the arabs and other muslims.
USA made terrorism, now they need to unmake it.