One of the mysteries about the ongoing offensive in Mosul, where Iraqi security forces are now pressing into the northern, eastern, and southern edges of the city, has been the apparent decision to leave unattended the desert between the battlefield and Syria. Unless this was a baffling oversight, the 20-mile-wide corridor of desert seemed intended to give Islamic State fighters an escape route to the group’s strongholds in Syria, perhaps to limit the destruction in Mosul.Sun Tzu said, ""When you surround the enemy always allow them an escape route. They must see that there is an alternative to death."
The reason is not to 'reduce the destruction' of the city, although it would be nice if that occurred. The reason is to reduce their will to fight to the death. If there's no choice, every man will resist to the last to the utmost of his powers. If there is, an increasing number of men will opt out and take to the road.
There's a second issue, which has to do with American technological advantages, that I won't discuss for OPSEC reasons. Still, it's not that baffling why a route through the open desert was left free for Sun Tzu's purpose.
The rest of the article is worth reading, as it pertains to the history of Tal Afar in the battle of Sunnis and Shia. I always think of Col. McMaster and the 3rd ACR when I think of that place. Tal Afar was one of the places where we learned how to win against the insurgents' influence on the population. The Shia militas are not going to be able to make that strategy work, and they are not strong enough to effect the Sri Lankan solution to an insurgent population.