Courage
I think it bears mentioning that, in my view, the debate that's broken out in comments here and periodically elsewhere around the web as to whether or not it took any particular political courage for Barack Obama to oppose the war in the fall of 2002 is a bit irrelevant. Whatever you may say about Hillary Clinton, pro or con, she obviously didn't take the position she did on Iraq because of short term political calculations. Clinton wasn't up for re-election until 2006. For people in her position, the cynical calculus and the substantive calculus wound up giving very similar answers.
For Clinton, the politically smart thing to do was to make her best judgment as to whether or not a vote for war would look smart in retrospect, and vote accordingly. Someone in Obama's position didn't face any real political risks in any direction. But the only cynical reason to speak out strongly against the war would have been a conviction that such speaking out would look smart in retrospect. Basically, political and substantive judgments track very closely.
It's different for someone facing the Max Cleland scenario of a tough 2002 re-election battle where you might really think that an invasion would be a long-run disaster but that you had no choice other than to support it. Neither Clinton nor Obama were in that position. Both could have gotten away with saying or doing just about anything. But both were ambitious people looking to do things that would look smart in the medium- to long-run. And only Obama did, in fact, do something that looks smart in retrospect. To mention the book once again, one argument I make is that while it's hardly a law of nature that "good policy is good politics" when it comes to something like Iraq it's really difficult to get the politics right in a vacuum. It makes a ton of sense, even in the most cynical possible terms, to try to build your political strategy on a foundation of sound substantive judgment.