A complete and utter nightmare. Iran has three times three times the population of Iraq, and a more stable(ish) government. The people wouldn't be as welcoming of the U.S. as the Iraqis initially were. Add in the same incompetent leadership as during the Iraq War (Rumsfeld, Wolfowitz, and Bremer), and disaster will ensue.
To effectively police the country, the U.S. would need to provide 1.5 million troops (using the ration of 1 soldier for every 50 Iranians). At it's peak, the entire U.S. military was only 2 million troops. Additionally, the same incompetent leadership that I mentioned above would push for a lighter "footprint", and not provide the proper amount of troops to secure the country (not that we would have them to provide).