“Fighting in Mosul is like working in a minefield,” said Sadi Ahmed Pire. Pire, now a Kurdish politician, commanded Kurdish forces as they fought to take Mosul from Saddam Hussein’s army in 2003.

While the city is home to Christians, Kurds, Shi’ites and other minorities, it is majority Sunni Arab and many in the city welcomed the Sunni ISIS fighters after years of neglect and oppression from the Shi’ite-led government in Baghdad. There are some indications that the population has now become resentful of ISIS’s restrictive rule, but communications to the city have been mostly cut-off for more than a month and intelligence about the city is limited.

“The single most important aspect of insurgency warfare is control, or support, of the population,” said Christopher Harmer, a senior analyst with the Institute for the Study of War, who completed several tours with the U.S. army in Iraq. “One of the reasons that ISIS had such an easy time in conquering Mosul was that the existing Iraqi Security Forces…had lost the support of the local population.”