First there was the "War on Crime" which had mixed results. Many Mafia leaders were caught and prosecuted, but crime is still a problem in many cities.
Then there was the "War on Drugs" which has filled many prisons. But drugs still come into the country and find buyers.
Now we have the "War on Terror". Some think it is succeeding: "We will keep the enemy on the run." (George W. Bush, October 19, 2006) Unfortunately, he thinks the enemy is in Iraq and Afghanistan, but it is global and growing larger. In fact, the war in Iraq probably helped al Quaeda grow. See "Al Qaeda Strikes Back", Bruce Reidel, Foreign Affairs, May/June 2007.
With the military already stretched thin, Bush is giving hints that he would like to attack Iran. Riedel writes that al Qaeda would like to see Bush attack Iran. Consider that the Iranian weapons found in Iraq may have been smuggled in by al Qaeda with the hope of proking the U.S. to attack Iran.
Would the U.S. leaving Iraq strengthen the terrorists' position or weaken it? If the U.S. leaves, Iran may come in to protect the Shia majority against the Sunni insurgents and the Sunni-based al Qaeda. A good general often lets somebody else do the fighting.