The difference between preventive war and pre-emptive war is a wide one that’s been well blurred by recent attempts. Iraq was preventive, not pre-emptive. Now pre-emption has become newspeak for preventive.
If Iraq had been an imminent threat to the US then the war was not an agressive war. It is part of a long sanctioned tradition of "preemption “Upon detecting evidence that an opponent is about to attack, one beats the opponent to the punch and attacks first to blunt the impending strike.”
As we all know, “For centuries, international law recognized that nations need not suffer an attack before they can lawfully take action to defend themselves against forces that present an imminent danger of attack. Legal scholars and international jurists often conditioned the legitimacy of preemption on the existence of an imminent threat—most often a visible mobilization of armies, navies, and air forces preparing to attack.”
The case can be made that the need to “adapt the concept of imminent threat” as laid out in the National Security Strategy Chapter V*, led to the actual adaptation the “concept of imminent threat,” in the national security strategy of the USA to include the threat presented by “rogue states and terrorists” and thus a change in what qualifies as preemption.
If Iraq was not an imminent threat to the US then the war was an example of a “preventive war”. Preventive war is based on the concept that war is inevitable and that it is better to fight now while the costs are low rather than later when the costs are high. It is a deliberate decision to begin a war."