On Monday, Iraqi troops—along with Peshmerga fighters and a Christian paramilitary group—launched a major offensive to take back the Iraqi city of Mosul from ISIS. The allied forces greatly outnumber their terrorist opponents, but that doesn't mean ISIS will give up its stronghold easily."They can make this a very tough fight," former CIA director and retired general David Petraeus told VICE News of the ISIS fighters. "There will be suicide bombers, there will be improvised explosive devices in the road, there will be car bombs."So far, ISIS has been putting up a brutal fight to hold on to the city. According to CNN, Mosul is currently the largest city under ISIS control, and is the last vital part of the caliphate's occupation in Iraq. ISIS has successfully held Mosul and its now roughly 1 million residents since first seizing the city back in 2014.But Petraeus, the retired four-star general who led US forces in Mosul in 2003, said that it's not a matter of if ISIS will be defeated, but when."These guys know they are dead men walking. The Islamic State knows they're going to lose," he told VICE News. "There's no question. They can even see the news reports showing the tanks coming at them, showing the tens of thousands of Iraqi forces."Check out our full interview with Petraeus on Wednesday's episode of VICE News Tonight on HBO, where he'll explain what it will take to establish stability in the region and talk about how important the fight for Mosul is in the greater war on terror. It airs tonight at 7:30 PM.