Now with Mosul, we're talking about 1M people. That's far, far more than the number of active ISIS fighters and other staff in the city. Have you never heard of guerilla warfare? If a population revolts, there isn't much a poorly-led and ill-trained military force can do to stop them. The people there are not revolting, so they're either cowards or complicit (and likely both). The people always have the ability to fight back, even if it has to be done as a war of attrition (use makeshift weapons to ambush small numbers of ISIS troops). Even in Nazi Germany and occupied France, there were resistance efforts from the local populations. Ever hear of the Maquis? ISIS doesn't have anywhere remotely near the resources and training that the Wehrmacht had in occupied France.