Iran is not Iraq, any war will see heavy casualties on both sides and could easily escalate into a global crisis. Iran, Israel and the US all know this, and for this reason, despite all the rhetoric floating around, I doubt we'll see a full scale military conflict, even if Iran develops and tests the evasive nuke, merely more noises.
The US will do what it has done traditionally in the case of strategically important countries with unfriendly leadership - bump off important people in the ruling party, fuel and arm dissidents against the Government, overthrow the leadership, and replace them with more US-friendly characters. That this has spectacularly backfired on more than one occasion isn't really going to change too much.
Only this will be difficult on Iran, because Iran is not like the rest of the middle-east. Any pro-democratic movements are nascent at best, and the people are deeply patriotic. Any military adventures by either side would be extremely short-sighted, and extraordinarily foolish.