An excellent way to clarify the justification for military action against the Iranian regime is to examine the crisis through the lens of the Just War Theory.

The JWT is a philosophical compass, orginially designed by two of the most brilliant Catholic theologians --St. Augustine and St. Thomas Aquinas-- as an ethical field manual for Christian soldiers. The driving concept of the theory is that a razor sharp line can be drawn between wars that are just and wars that are unjust.

Philosophers and theologians have further developed the jus ad bellum since the Catholic thinkers of the middle ages laid down the theory's groundwork. As the nuclear age dawned, 20th century thinkers revisited the works of Aquinas and Augustine, using the just war criteria to establish the Geneva convention (and notably the Hague convention in 1907).

That said, the rules for just war still apply today: a war is just only if it is waged by a legitimate authority, proper protections must be provided for civilians, the violence used in war must be proportional to injury suffered, and so the list goes on.

The original framers of the just-war theory never considered preemptive action, which is exactly the type of war that could be waged against an aggressive, nuclear Iran. Many critics of Operation Iraqi Freedom invoked the lack of preemption in the just war criteria to claim that the American invasion was, by definition, unjust.

Just War Theorists may not have specifically mentioned preemption, but the principles of just war --without a shred of doubt-- authorize action against the Iranian regime.

Here's why, principle by principle:

1. A just war can only be waged as a last resort. All non-violent options must be exhausted before the use of force can be justified. Roger that. Which is why we first explored the EU led diplomatic charge. Now that the European initiative has failed, we're taking a stab at UN sanctions. If those fail, and if there is a reasonable chance that Ayatollahs are close to a functional bomb, then all options have been explored and military force is justified.
2. A war is just only if it is waged by a legitimate authority. America is a recognized nation, whose armed forces comply with the laws of the Geneva convention. Check.
3. A just war can only be fought to redress a wrong suffered. If the Iranians ignore the directive of the United Nations, and the security of Europe, Israel, and the United States --not to mention the nations of the Middle East-- is threatened, then a wrong has been suffered and the military correction of that wrong is authorized. Oorah.
4. A war can only be just if it is fought with a reasonable chance of success. Uh yeah, check.
5. The ultimate goal of a just war is to re-establish peace. More specifically, the peace established after the war must be preferable to the peace that would have prevailed if the war had not been fought. Iran is a destablizing force in the region and not even the worst of Ayatollah apologists deny that fact. And although the word "preferable" is a bit ambiguous, the civilized world can agree that a free Iran is most certainly preferable to the totalitarian reign of Islamic fundamentalists.
6. The violence used in the war must be proportional to the injury suffered. Okay now I read this as authorization for preemptive strike to remove the Iranian nuclear capacity, not full-blown regime change. I suppose that you could argue that permanent "cure" for the "injury" of destablizing the region would be fostering a new Iranian democracy, but that comes off as a bit of a stretch (no matter how badly I want a free and functional Iranian republic).
7. The weapons used in war must discriminate between combatants and non-combatants. There is no other nation in the history of mankind that is more proficient in precision engagement than the United States. Check plus.

There are of course offshoots of the JWT. Marxists only believe that war is just if it is means to progressive ends, pacifism is the antithesis of just war (i.e. war is never just), and Islamic fundamentalists use an almost bizzarro version of the just-war theory in their fatwas. In fact, Islamic jihad is by definition a complete perversion of the jus ad bellum, e.g. killing innocents is authorized, no legitimate authority is necessary, war is fought with little chance of success, war is not used as a last resort, and war is used to establish fundamentalist dictatorships instead of peace.

Should the time come to bring down the thunder on the Iran nuclear infastructure, it is only appropriate that the Just War Theory be invoked against the Islamofascists who have dishonorably broken the long-standing treaty of ethical warfare.