Let's see. What would a war against Iran look like?
Well, the USA has bases in Iraq and Afghanistan, and essentially control of the Persian Gulf and straits of Hormuz. Even though Russia has ties to Iran (and thus will probably not support us), we essentially have all bases covered; Iran is surrounded.
The key from the US point of view is to minimize American troop involvement and limit it to an air campaign. Thus, we would probably do our best to smash the Iranians from the air, but since I doubt that would knock them out, a war on land is going to happen regardless.
Now, I think the best source of an ally in this struggle is Pakistan. Assuming we were to support Pakistan, they would probably make significant gains against the Iranians, though they would also take heavy casualties. "Coalition" forces might advance from Afghanistan, but since the US is probably not going to get a full commitment from any nation, it would be doubtful they would support another war... We won't advance very far from Afghanistan.
An attack by the Iranians into Iraq will force the US to intervene, and a large mixed armored and infantry force will be pressed against them; it would be justified by the "Iranians attacked first" argument that comes from their attack into Iraq. We could probably take many of the major oil fields, though guerilla resistance will be heavy, the Iranian military will fight fanatically for every inch of ground, and I think some "excess Soviet equipment" acquired on the "black market" (read: Russia supplying Iran) will end up in their hands...
I don't know if the US drive will actually reach Tehran, but it might not need to; the Iranian government might be toppled by the Westernized youths or a military coup.
Don't go celebrating then, either, though, for a more Westernized Iran would still be infuriated at the US troops on its soil, still fight fanatically, and probably have the backing of most of the world.
The prospects are that the US will certainly not lose the war conventionally speaking, nor will it lose much of Iraq, but it will be a stalemate militarily, and a vast loss diplomatically and politically. Look for the Dems to win the next election if the US goes to war with Iran.