"Urban warfare remains characterized by slow, massive destruction. Yet 50 years ago, there were no computers, no internet, no GPS, no UAVs, no digital communications, no night-vision devices, and no precision strikes. Two facts account for the lack of change in tactics. First, cities are constructed of steel and concrete, with streets providing the open spaces, which are usually linear. Any fighter in the open is quickly cut down. No technology can accurately detect and count humans inside buildings and tunnels. So the attacker must advance by blasting through the sides of buildings and slowly, slowly search every room. Second, tens to hundreds of thousands of civilians can be trapped in the cities. The terrorists in Mosul have prevented the civilians from leaving in order to use them as shields."