Just to add my $0.02, obviously there was and is no evidence linking Saddam to 9/11.
But as to what led to the war, well, the tale is simple and tragic. Bush planned on invading Iraq before becoming president. There is documentary evidence to that effect:
Bush Planned Iraq 'Regime Change' Before Becoming President
Even post 9/11, it was generally accepted between London and Washington that the "War on Terror" was just a pretext, and that the intelligence was being retrofitted to the policy. There is more documentary evidence than you can shake a stick at:
The Downing Street Memo :: What is it?
A sad tale indeed. Sadder still is that neither Bush nor Blair will face prosecution.