With terrain. Iraq is flat. Iran is more like Afghanistan than Iraq. It's much easier to bury and hide things in the side of a mountain than to go down in a plain.
My suspicion is that bunker busters will cause delay in Iran's program, but not shut it down unless we create sufficient chaos within the country to bring down the regime. The problem there is that Europe, Russia, and Asia are just craven enough to keep it afloat. That leaves total war as the only option. Iran it a big place. We've lost our basing in Iraq and would get very little other local support. We're out of money, our equipment is tired, and our credit line is toast.
Bush blew it big time when he stopped in Baghdad to 'fix what we broke.' He should have immediately gone to work destabilizing Tehran. It may still be possible, but it will involve protracted ground work that gives Iran time to go full nuclear with the demonstrable willingness to use it. Can we buy that time with a bunker buster? Don't know.
You?
The MOP is one heck of a weapon. Seen it during testing. Awsome capability and very ‘smart’ when it comes to weapons effects.
If we run in and “MOP” ‘em and while we are there we knock the pee outta their “leadership” (political and military), and then leave with a warning that we will come back and drop more on their head if they don't knock it off. . .and NOT care if they collapse and NOT do the usual re-built them after we destroy them.
But that's just me. . .