The difference between preventive war and pre-emptive war is a wide one that's been recently blurred by certain parties. The invasion of 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."
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."
Pre-emptive strikes need no justification. They've been recognized as legit for centuries. Invading Iran would be preventive, not pre-emptive.