The battle for Mosul represents the future of warfare—and it wasn’t pretty for America’s allies. A ragtag army of a few thousand Islamic State fighters managed to hold the city for months against some 100,000 U.S.-backed Iraqi security forces. The ISIS fighters communicated via social media and were armed with crude explosive devices and drones available at Wal-Mart. In the end the rebel fighters were dislodged, but not before an estimated 7,000 people were killed and another 22,000 wounded.

U.S. commanders ought to imagine...