There has been considerable speculation about how hard Islamic State fighters would resist: Would the militants make a final stand in the villages or pull back to Mosul to fight another day? The sounds of battle Monday morning indicated there was resistance.

Attack helicopters could be heard overhead at the start of the assault, followed by the thud of tank rounds as the pesh merga fired on Islamic State positions across a stretch of the Nineveh Plain. There were bursts of machine-gun fire. A powerful airstrike sent out shock waves. In the distance, there was a funnel of black smoke.

Pesh merga officers said they expected to be joined in a couple of days by Iraqi forces, which would help them secure their gains and, ultimately, push farther west. But first there were villages to take and secure, and the goal for the pesh merga on Monday was to take control of more than 45 square miles.

The battle plans have unfolded in recent weeks against a backdrop of grave concerns about the civilian population in Mosul, which by some estimates numbers more than one million. United Nations officials have warned that 200,000 people could be displaced in just the first few days of an assault on the city, and humanitarian groups have scrambled to set up emergency camps for the displaced.