What would be the endgame of going to war with Iran?

There are a lot of drumbeats to push America to go to war with somebody -- with the usual suspects being North Korea and Iran.

It seems that the model for how a war with Iran would go would be the previous war with Iraq, but there are some significant differences. For one, Iran covers about four times the geographic area, including much more mountainous terrain (and in fact, more of every kind of terrain). For another, Iran has much larger population, over 80 million people. And on top of that, Iran has a much larger and more modernized military than Iraq ever had, and is not diplomatically isolated, enjoying fairly strong alliances with nations including Russia (which could thusly be drawn into such a war) and China. It seems exceedingly unlikely that the same nations which joined the US for its military adventure in Iraq (especially the UK and Australia) would be game to go it again on a 4x larger scale.

So what would come of an invasion attempt? What resources would need to be mustered and where would they be drawn from? Where would we be after a year of such a war, or five, or fifteen?