Well, that's part of it, but not nearly the main part. Think about it: now that Afghanistan has been neutralized, who are the three biggest supporters of anti-Western terrorism in the world?
I would say Iran, Syria and Saudi Arabia.
Each of them shares a long border with Iraq. Once we occupy Iraq, we have all three within a very short reach of a substantial American force. Iraq is a pawn (OK, maybe a bishop) that just happens to be sitting on a very useful square.
Judging from the press- newspapers, wire services, networks, talk radio, internet- almost nobody has grasped this.