At the point where remaining there is no longer is America's best interest. For the foreseeable future, the US needs a middle-east presence. Winning the war will result in the majority of our combat forces being removed, but some numbers will and arguably should remain for as long as the mid-east is a strategic concern for the USA.
We already created a presence when we invaded Afghanistan. Didn't need to go into Iraq, except to kick Saddam out of office and take over his oil fields (and banks).
As I've been intimating, there never will be a 'victory' in Iraq. It's all an illusion.
Invading Iraq had nothing to do with the 'War on Terrorism' and everything to do with securing a middle-east source of oil. You said so yourself.
Now, if only Bush were as honest as you.