Michael Walzer, an important American political philosopher, outlines the difference between preventive war and preemptive war in his book Just and Unjust Wars:
A preventive war is one in which a nation attacks against a possible threat to the balance of power relations.
A preemptive war is one in which a nation attacks against an imminently possible threat to the peace of a nation.
This raises the question, since the book was written after Vietnam — a preventive war — whether the War in Iraq would count as either a preventive or a preemptive war. There are time variables, however, because at the time of the invasion of Iraq, the war was advertised and buttressed as a preemptive war: a war that if not fought, it was said by the senior cabinet members of the administration that the United States would see the evidence of the weapons of mass destruction “in the form of a mushroom cloud”.
However, that justification — or the truth of the WMDs — turned out to be false. The administration must have known it was false all along, since the informants from the IAEA and even the CIA agreed there were no weapons of mass destruction. This information was systematically silenced and manipulated. This bit of evidence raises other important questions. Seeing as the evidence for a first strike was decidedly minimal, was the implicit justification for war preventive rather than preventive? Was the real justification simply that the United States did not want to see the balance of power relations in the region change over time? With long-term interests in the region, and a mercantilistic urge to control the world’s natural resources, did the United States lie in order to wage an aggression preventive war?
This is what the evidence has suggested. However, justifications for war have changed over time. The administration no longer says we have invaded Iraq because of the imminent threat of weapons of mass destruction. Instead, the justification for staying in Iraq today is undeniably a preventive one. The argument that Iran should colonize and take over Southern Iraq is a preventive justification for the furtherance of war. The argument that the Turks, acting preemptively to end Kurdish insurgent attacks, would cause a balance of power crisis in the region is likewise a preventive justification for war. The argument that the Sunni Triangle has as its aim to destroy Shia populations and rule all the governates of Iraq is also a preventive claim for war.
The point that Walzer is trying to make in his book Just and Unjust Wars, however, is that no justification for preventive war is justified. The principles of Western liberalism, using the term in the broadest sense, have long held that wars of aggression are unjust. A preventive war, while it may be noble in its efforts to end the spread of violent ideology, is itself a source of violent ideology. Since preventive war uses violence and militarism as its only means to change policy and affect the balances of power, it is an unfounded doctrine in the Western liberal tradition. Wars of aggression justified by a preventive strike are unjust wars.
What about a preemptive strike? What lawyers call “hostile acts short of war” Walzer says does not count as a warrant for a preemptive strike. Even if these hostile includes acts of violence, this may not be seen as a intention to make war. Troop movement, border incursions, naval blockades, military alliances, etc. are all hostile acts short of war, yet they do not constitute the intention to make war itself.
In general, however, where the justification about preemptively striking is most important is not regarding imminent threat (as it is so commonly thought to be) but on sufficient threat. And according to this set of criteria, outlined in the book, the invasion of Iraq is not justified.