Excellent counter-argument you bring up. The real question would then be whether or not the war is just (I do believe that sanctions intense enough to provoke a revolution is an act of war, especially if purposefully aimed towards it).

St. Thomas Aquinas has three conditions which need to met before a war can be called “just”:

1. Does the prince who declares war have the correct authority to do so?
2. Does the war declaring nation have just cause to seek redress for an injury done to it?
3. Does the party declaring war have just intent in promoting good or avoiding evil?

Ignorant as I am, I'll try to answer these questions:

1. Yes, the federal government has the authority to declare war (or enact/influence acts of war by today's modern forms of warfare).
2. Who knows who deserves what at this point in the middle east. Redress of injury isn't the goal so much as making Iran predictable.
3. I don't know. It's either a desire for stability or a desire to consolidate power. Iran is either a danger worse than North Korea that would use nukes suicidally if they had them, or a country that wishes it had deterrents to safeguard itself from the west.

Answers to questions 2 and 3 aren't clear. That's all I got.