In the battle against the Islamic State for Mosul, Iraq’s second-largest city, there is no continuous front line, but a patchwork of battlegrounds in the city and all around its edge.

When the Islamic State, also known as ISIS or ISIL, retreats from a position, it tries to leave as much damage behind as possible, including burning oil field wells to provide concealment ahead of a government advance. Snipers are also in place, ready to strike.

A village or neighborhood that is retaken by government troops could soon be flooded again with extremist fighters, who are rarely far away.

I recently spent three weeks in this volatile zone, first with the Iraqi Army in eastern Mosul and then, after setbacks for the army, whose block-by-block battle of attrition was not going the way it had hoped, with units of the Iraqi federal police.

The police, traveling in armored vehicles, are playing a significant role in the battle for Mosul, sometimes supporting the army. They are also responsible for holding their own territory around the city.