Mosul shows difficulty of removing Islamic State from urban area

[...] the growing controversy over the high number of civilian casualties believed caused by recent U.S. air strikes has touched on a major part of the answer:

After a March 17 explosion that residents say killed at least 100 people in Mosul, the U.S. military acknowledged an air strike was involved.

[...] the top commander of U.S. forces in Iraq said investigations may reveal a more complicated explanation, including the possibility that militants rigged the building with explosives after forcing civilians inside.

Army Lt. Gen. Stephen Townsend said recent civilian casualties in Mosul were “fairly predictable” given the densely populated urban neighborhoods the Islamic State fighters are defending against Iraqi troops.

Over the past 2½ years, Iraqi forces backed by U.S. special forces and coalition air strikes have managed to push Islamic State out of most of the territory they overran in the summer of 2014 — retaking three major cities and numerous smaller communities.

In Tikrit and Sinjar, Islamic State let the population flee early on, allowing Iraqi and coalition forces to liberally use air strikes and artillery to retake the areas by the autumn of 2015.