Once we leave, they can send militias out to patrol the border. Though, we can't even secure our own border. What makes you think we will EVER be able to secure Iraq's border?
In which case the terrorists have no need to attack in Iraq. They get to Run around the madrassas screaming "We defeated the Americans join us in Jihad" and recruit the next generation of "Holy Warriors" to plan the next series of 9-11 style attacks. We need to draw the "Holy Warriors" out of hiding into a kill zone where their weakness (Convectional military power) is projected up against our strength (firepower, firepower, firepower). Iraq is about taking and maintaining the initiative in the War on Islamic Terror. Iraq is what Sze Tse called "Death Ground" a battle field the terrorists MUST fight on for political reasons. We cannot defend everything, appease or isolate ourselves from these people. I thought after 9-11 even the most cast iron conservative would realize that! Isolationism has NEVER worked. We must fight these people! Iraq is the IDEAL place to do it.