A military analyst told Mattis in 2011 that if US troops left Iraq 'all hell's going to break loose'

Defense Secretary James Mattis was opposed to President Barack Obama's full withdrawal of US troops from Iraq in 2011. Mattis' disapproval stemmed in part from an intelligence briefing he received in November 2011, in which he was told that if all US soldiers exited Iraq, "all hell's going to break loose" by the summer of 2014.

"I asked, 'What happens if we pull our troops out?'" Mattis, who was then head of US Central Command, told The New Yorker's Dexter Filkins. "The analyst told us that another group would appear. And, when it appears, it will be more vicious than any you've seen yet."

He added, "She finally blurted out, 'General, if you pull all our troops out, then, by the summer of 2014, all hell's going to break loose.'"

Mattis said that the total withdrawal of the on-the-ground American military presence destabilized Iraq and allowed the Islamic State, also known as ISIS, to take over significant portions of the country.

ISIS emerged as an unexpectedly powerful force in late 2013 and seized the city of Fallujah, Iraq in January 2014, the same month that Obama infamously described ISIS as the "JV team" to The New Yorker.