Richard Haass in Project Syndicate:
Whatever progress had been made in Afghanistan after the US invaded and removed the Taliban government that had provided a safe haven to the al-Qaeda terrorists who planned and carried out the 9/11 attacks, it was deemed inadequate. Many in the Bush administration were motivated by a desire to bring democracy to the entire Middle East, and Iraq was viewed as the ideal country to set the transition in motion. Democratization there would set an example that others across the region would be unable to resist following. And Bush himself wanted to do something big and bold.
I should make clear that I was part of the administration at the time, as the head of the Department of State’s Policy Planning staff. Like virtually all my colleagues, I thought Saddam Hussein possessed WMDs, namely chemical and biological weapons. Even so, I did not favor going to war.
More here.