Senator McCain Clearly States It

This is why we are in Iraq. It's the main reason we went in and the main reason we can't leave until the job is done. Every time the subject of getting out of Iraq comes up, this is what needs to be said.

"If we fail in Iraq, it will be cataclysmic. You'll see factionalization and eventual Muslim extremism and terrorist breeding grounds that would, I believe, pose a direct threat to the security of the United States. And I'm very glad that the American people -- understandably dissatisfied, understandably frustrated -- still, the majority of them don't think we ought to cut and run."

This is the "constitutional" reason we are there. "Spreading Democracy " is our tool. "Freeing people" is a result. (Powerline)