Washington would neither have to foot nearly as much of the bill nor commit as many soldiers as it did in Iraq, but there are other aspects of a military engagement that must be considered: What happens if the North falls but the war turns asymmetric?
There are two distinct reasons why an armed attack on Iran is illegal under international law, whether the attacker is Israel or the United States.
The USA has always had a number of nationalist/colonialist/expansionists within the reaches of power in Washington D.C., but
Some have argued that the Cold War-era mutual assured destruction doctrine would contain Iran. That is a big assumption -- specially in light of the Iranian regime's ideology.