A simulated napalm explosion. Until the Vietnam War, few objected to its use; today it is considered a war crime © Andrew Pendracki

On the night of 9th March 1945, 339 American bombers took off, heading for Tokyo. Their payload: incendiary bombs filled with a gelatinous mixture of coconut oil soap, aluminium naphthenate and gasoline, developed just a few years before at Harvard University. The B-29 flew in at low altitude, and dropped 690,000 pounds of napalm within an hour. Tokyo’s paper and wood houses were soon engulfed in flames. Explosives merely kill those within the range of the bomb’s impact. Incendiary weapons spread death more widely since their targets provide the fuel for their propagation.

The bombing created a firestorm. Gale-like winds swept through the streets of Tokyo, sending a column of fire 18,000 feet into the sky. Flying high overhead, American airmen could smell the stench of burning flesh. Not a building was left intact over 15 square miles of the city. Probably over 100,000 Japanese men, women and children died that night.

The Tokyo raid was seen as a great success in Washington. For the next ten days, until stocks of napalm ran out, an incendiary bombing campaign ravaged all the major Japanese cities (other than Kyoto). “It was nothing short of wonderful,” a general in charge of the campaign crowed, that the American air force had meted out the greatest damage ever “inflicted on any people in a single eight-day period.” Napalm killed many more Japanese than the atom bombs, and probably did as much, if not more, to destroy their will to keep fighting.

Robert M Neer’s thoughtful history, Napalm: An American Biography (Harvard University Press, £22.95), is divided into three sections: hero, soldier and pariah. These neatly describe the evolution of our attitudes towards the incendiary. During the second world war, few had any objections to its use. Its horror was viewed as an advantage because it frightened opponents. Battle hardened soldiers who resisted conventional attacks soon surrendered after being sprayed with napalm from flamethrowers or from specially adapted tanks. Our atavistic fear of fire made napalm the weapon of choice. “People have this thing about being burned to death,” one soldier explained.

Napalm was also decisive in Korea, proclaimed “the number one weapon” by the New York Herald Tribune. Tactically, it slowed the North Korean invasion and it protected the flanks of US marines during the Inchon amphibious landing. It was “the most outstanding single weapon employed in the Korean operations,” according to one Pentagon staffer. The implication is that without napalm, the North and their Chinese allies would have conquered South Korea. Incendiary bombing raids also devastated Korea’s cities. After repeated bombing Pyongyang, a city with a pre-war population of 500,000, was said to have only two buildings left standing.

America dropped 32,357 tons of napalm on Korea, twice as much as it dropped on Japan in 1945. But Vietnam took the heaviest hit: 388,000 tons of napalm between 1963 and 1973. And it was Vietnam that made the incendiary a byword for cruelty. Before then, napalm was accepted as a useful if brutal tool of war. By 1966, activists began a boycott against its maker, Dow Chemical. In 1967, articles in mainstream magazines informed Middle America that “napalm, far from the modern marvel described by earlier correspondents, was a diabolically cruel child seeking killer.” It was reading one of these articles that convinced Martin Luther King to campaign against the war in Vietnam.

Napalm sticks to skin. Even a drop of it is unbearably painful. As an incendiary, it is imprecise and likely harm innocent civilians. That was true in Japan and Korea. Why was it Vietnam that turned public opinion so strongly against the weapon? Neel suggests that defeat in Vietnam made napalm seem less heroic. I suspect it has more to do with the way the war was covered.

We all recall the famous photograph of a naked nine-year-old girl running down the road, screaming after a South Vietnamese napalm attack on her village. This provokes a very different reaction than a print article coolly describing how napalm prevented a Chinese attack on American troops. Photographs and television are media that emphasise the pain done the victim. They are by nature empathetic. Print is colder. Lists of the 50,000 casualties on the first day of the battle of the Somme did little to turn the British people against the first world war. But video of a crying family as they hear of one squaddie’s death makes most of us instinctively want to pull our soldiers out of Afghanistan.

Or perhaps we have just become more civilised. Today, the use of napalm is considered a war crime. When the US used firebombs against Saddam’s army in 2003, the Pentagon vociferously denied that it was napalm. It later explained it was another incendiary that did not yet have such a bad reputation.