“The bomb falls closely behind us, and I feel its fiery breath,” a Viet Minh officer recounted in a diary passage quoted in Bernard Fall’s history “Street Without Joy.” “The men are now fleeing in all directions. . . . I stop at the platoon commander . . . his eyes were wide with terror. ‘What is this? The atom bomb?’”

In a short time, however, the psychological impact of the pyrotechnics of burning gel waned. To the Vietnamese, napalm became just another kind of bomb, deadly and destructive to be sure, but no more frightening than any other air-delivered weapon. The French increased the use of napalm in subsequent battles, but by the time the French army surrendered in 1954 at Dien Bien Phu, the Viet Minh had learned that napalm, like all airstrikes, could be avoided through concealment, dispersion and entrenchment.

Right now, the Islamic State is going through this same evolutionary process along the western border of Kurdistan. As one can see from the grainy black-and-white “gun camera” tapes released by the Pentagon, the U.S. airstrikes of the past week have mostly succeeded. Lazy and overconfident Islamic State forces positioned their artillery and mortars in the open. Their convoys were bunched together and tended to travel along established highways, where they could easily be spotted by drones. As planes approached, fighters cowered in fear or ran away.