With support from American air strikes and special operations, Iraqi forces have launched the battle to retake Mosul from the Islamic State (ISIS). The fighting promises to be difficult. Though the Iraqis estimate that there are no more than about 5,000 ISIS fighters in the city, ISIS has had more than two years to dig after taking the city of in June 2014. American and Iraqi officials have warned it could take weeks or even months to liberate Mosul. The real danger for the United States, however, is what happens after Mosul.