Is, of course, its oil:
Iran's production capacity represents about 5% of the world total. Mr Hamilton notes that supply disruptions of that magnitude in the past were associated with oil price increases of between 25% and 70%—and with American recessions. … So one interesting thing to note is that Iran could potentially send America into recession all by itself, simply by halting its oil production for a few months. That wouldn't be good for the Iranian economy, of course, but perhaps that's a small price to pay for the smiting of one's enemies, and so forth. America couldn't easily respond with force as it would in response to, say, a nuclear attack. Closing the Strait of Hormuz would be a different geopolitical animal but would, if successful, bring the global economy to its knees.
The Dish 