Getting them out of the city, though, will be difficult, and thus far, Islamic State fighters have chosen to stay and fight rather than flee. Iraqi forces will have to fight block by block and street by street against an enemy that has had ample time to prepare its defenses and has learned from previous battles in Manbij in Syria, Sirte in Libya, and Fallujah in Iraq. The fight for western Mosul will take time and will further stress Iraqi forces exhausted from a hard fight for eastern Mosul.

Yet the Islamic State is essentially surrounded in Mosul—a condition that's presaged its defeat in other cities as well. Why haven't they fled before now? It’s anyone’s guess, but it’s worth remembering how we—the U.S. military and its partners—often assume the enemy understands his situation better than he actually does. Because we usually have good situational awareness, we assume the other guy does as well. But if we try to put ourselves in the shoes of the enemy amidst the fog of war, it’s very possible the Islamic State fighters simply failed to realize how cornered they were until it was almost too late.

A phenomenon whereby fighters fled a city while leaving a rump force behind has more or less repeated in places like Manbij, Sirte, and Fallujah until now. What makes 2017 different from last year is there are very few places to which the Islamic State fighters can still retreat. Aside from a few remaining strongholds in Iraq—most of them along the middle Euphrates River Valley—and a few towns in Syria such as Raqqa and Deir Ezzor, Islamic State fighters have few options for places to go.

One option remains the desert caves west of Mosul, while another would be the remaining cities under Daesh control in central Syria along the Euphrates River. The Russians and Assad regime, in particular, fear the Islamic State will retreat to the latter, where a small regime garrison hangs on by its fingernails outside Deir Ezzor.

Throughout the fall of 2016, Russian generals with whom I was negotiating in Switzerland on the fate of Aleppo would anxiously inquire about our plans for Mosul and Raqqa. They had not been successful in convincing either Iran or Hezbollah to devote larger numbers of fighters to the defense of the besieged regime garrison. Indeed, they had not been successful in convincing any of their coalition partners—Iran, Hezbollah, the Assad regime—to focus on fighting the Islamic State, which is just one of many reasons why partnering with Russia in Syria remains folly. At the same time in which U.S.-allied forces were dealing the Islamic State defeat after defeat, Russian-backed forces repeatedly failed, with much embarrassment, to even defend the isolated desert town of Palmyra in central Syria just a few hours drive from Damascus.

Ideally, of course, the Iraqis would be pressing the Islamic State in Mosul at the same time in which Russian-backed forces were doing the same in Deir Ezzor, and U.S.-backed Kurdish forces were doing the same in Raqqa. Simultaneous pressure against the Islamic State—pressure that creates dilemmas for where the Islamic State’s commanders devote their precious resources—won the gains of 2016.