"We're fighting the terrorists there so we don't have to fight them here." This sounds eerily like the "domino theory"; we had to contain the Communists in Viet Nam or else we would be fighting them on our shores.
But terrorists will not be contained by an army - people in London and Madrid found that out the hard way. If there is a current terrorist plot against the U.S., it won't necessarily be planned in Iraq or Afghanistan. Why run the risk of disruption? Let's see, it could be planned in Pakistan, Iran, Saudi Arabia, Egypt, Germany. Germany? Yes, Germany, remember some of the 9/11 attackers were in a cell in Hamburg.