(A) political leaders aren't supermen and -women. Like all of us, they have to make the best decisions they can with the limited information available to them at the time;
(A1) [ADDED:] the signal-to-noise ratio can be problematic; the available bits of information are often of varying quality and sometimes are flatly contradictory --- a major part of the leadership challenge is figuring out what the hell is really going on;
(B) most political leaders genuinely want to do a good job, even if that's mixed in with a larger- or smaller dollop of self-interest (as is the case with most of us);
(C) in late 2002 and early 2003, memories of 9/11 were still raw;
(D) Saddam Hussein had irrefutably demonstrated that he was willing to use weapons of mass destruction in pursuit of his ambitions: he had used chemical weapons both on Iraqi Kurds and on Iranians (and let's not forget his brutal conquest of Kuwait);
(E) it was unclear to what extent Hussein had made any progress on building -- or buying -- nuclear weapons;
(F) the downside of a false negative on that issue was considerable; and
(G) hindsight is 20-20, and there are always Monday-morning quarterbacks around who are certain they could have done better.