Here's a blast from Reason's past: An online debate over invading Iraq between political scientist John Mueller and Contributing Editor Brink Lindsey. This originally took place in late fall of 2002 and posed the question:
In the wake of the 9/11 terrorist attacks, the Bush administration has articulated a new foreign policy for the United States. The "containment" of hostile states has been replaced by a policy of military "pre-emption" and "defensive intervention," which sanctions U.S. military action even against states that are not imminent threats. War with Iraq may be the first major expression of this new policy (as of press time, no shooting had yet begun).
Is such a preventive war justified?
The debate is here.