Point of Clarification
I'm manifesting the common character trait of Soldiers who are aware they may find themselves in another war.
In a war, the casualties inflicted are in direct proportion to the amount of time people are shooting at each other. This is not speculation, but fact. If a war is shorter, fewer people die.
I prefer to win wars rather than lose them. History shows that it sucks rocks to lose wars. Winning isn't necessarily a walk in a park, but I think we can take as a given that it beats losing. Ask the Germans.
So from these two characteristics, we can conclude that ideally, you open a war with an incredible, overwhelming horrific demonstration of violent intent and capability that will make the enemy go, "Hot damn, let's negotiate NOW!"
For instance: If we hadn't nuked Japan, they would have had to be invaded. 1 Million US troops estimated to be killed, and the Japanese casualties would be literally incalculable, likely to the point that Japanese culture would be exterminated utterly.
In today's political climate, that's pretty much only possible by nuking the holy living hell out of someone.
Fighting the Iranians would be pretty simple. The Revolutionary Guards and Iranian Army would fall apart in 30 days or less. The Iranian Navy gets sunk in the first 24 hours, and that's only because the USN can only reload their missle launchers so fast.
Holding Iran would be a stone-cold nightmare. Imagine Iraq, times three in both population and land area. Simply impossible. The only way to have a Happy Ending would be to:
A) Nuke it into submission, so that we really no longer care what happens in the Land of Stone Age Primitives
B) Do an Afghanistan-style backing of a faction with significant military power and provide forward air controllers to allow them to blitzkreig to Tehran and install a new government.
Will the Real Iranian Resistance please stand up? Please stand up?
For extra points, you really need a significant part of the Iranian Army to back this faction (or at least sit out the civil war), or it will be nothing more than a protracted, messy civil war which will at least prevent the Iranian government from having the spare effort to dink with Iraq or Lebanon, but which will not look terribly good on the Evening News (this being the driving factor in American foreign policy).