Things are not quite that simplistic. We live in a world of proxy wars now. Iran has powerful allies, enemies of America with massive nuclear capabilities. And it wouldn't be "us" going nuclear first. Bottom line... it's not worth taking that risk of starting that last war. Not time yet.
We throw down on Iran, no one is going to stand up for them in any serious fashion. The only thing that's going to happen is a lesson will be taught that we're done with third world posturing. Iran is a convenient tool as a thorn in our side, but only until we decide to remove it.
I'm not a fan of our initial misadventure into Iraq, but did any of their "allies" stand up and fight back? What did Quidaffyduck do when we showed the resolve to take out saddam? IIRC, they sent their nukes to North America.