What could Iran do? Plenty. Send Revolutionary Guards into Iraq to make that country a worse hell for the 135,000 U.S. troops. Incite Hezbollah to launch rockets on Israel to widen the war. Attack U.S. allies in the Gulf. Encourage the Shias in Iraq and Saudi Arabia to attack Americans. Mine the Strait of Hormuz. Activate Islamic loyalists to bring terror home to the United States.
In short, a U.S. attack on Iran could lead to war across the region and interruption of the 15 million barrels of oil a day that come from the Gulf, which would drive the world economy into instant cardiac arrest.