Four twin-engined transports fly line abreast, eighty metres apart, a hundred and fifty feet above the canopy. Behind each one a white plume spreads and settles into the trees. The run takes about four and a half minutes and covers sixteen kilometres.

The crews called themselves Ranch Hands. Their unofficial motto was a joke on the Smokey Bear campaign every American child knew: Only you can prevent a forest.

Between 1961 and 1971 they sprayed roughly twenty million gallons of herbicide over Vietnam. The argument about what that did to the people underneath, and to the men in the aircraft, has now run four times longer than the war.

What They Were Trying to Do

The military logic was not complicated. The Viet Cong and the North Vietnamese Army moved and hid under triple-canopy jungle. Remove the canopy and you remove the concealment. Destroy the crops and you remove the food.

The C-123 Provider was chosen because it could fly low and slow with a heavy load and get into rough strips. Fitted with a thousand-gallon tank and wing booms, one aircraft could treat a swath eighty metres wide and sixteen kilometres long in under five minutes. They normally flew three to five abreast, at treetop height, straight and level, over territory held by people who were shooting at them.

Ninety-five per cent of all the herbicide used in the war went out this way. The rest went by hand sprayer, truck, helicopter and boat, mostly around base perimeters.

That ITN report was broadcast on 1 March 1967 and includes interviews with the aircrew flying the missions. It is worth watching for one reason above all others: the scientific objections it records were being made at the time, not in hindsight.

The Objections Were Contemporary

The Federation of American Scientists objected in 1964. The American Association for the Advancement of Science called for a field investigation in 1966. In 1967, seventeen Nobel laureates and five thousand other scientists petitioned for an immediate halt.

In April 1970, after a Bionetics study found 2,4,5-T caused birth defects and stillbirths in mice, military use of it was suspended inside the United States. Spraying in Vietnam continued into January 1971.

The dioxin numbers have been revised upward since. The 2003 study by Stellman and colleagues in Nature, working from recovered mission records, found the mean TCDD concentration in Agent Orange was closer to 13 parts per million than the 3 ppm previously assumed, and located records of some two hundred pre-1965 missions that had sprayed over two million litres of Agent Purple, an earlier and more heavily contaminated formulation. Their estimate of total dioxin applied — 366 kilograms — roughly doubled the previous figure.

The Admiral and His Son

The clearest account of what this cost belongs to one family, and both of them put it in writing in The New York Times Magazine in August 1986.

Admiral Elmo Zumwalt Jr. commanded US naval forces in Vietnam and ordered the defoliation of the Mekong Delta riverbanks to protect his boat crews. His son, Elmo Zumwalt III, served on those boats. The son developed lymphoma and Hodgkin’s disease and died in 1988, aged forty-two.

The admiral also explained the decision itself, and it is a more ordinary sentence than people expect: they asked the Army and Air Force whether Agent Orange could hurt people, were told it could not, and believed them.

One correction to how that story usually circulates: Zumwalt Jr. did not order the Agent Orange programme. Ranch Hand began in 1962, six years before he took command in Vietnam. What he ordered was the riverbank spraying in the Mekong Delta.

The Aircraft Kept Flying

The part almost nobody knows is that the story did not end when the spraying stopped.

Contaminated UC-123Ks were passed to the Air Force Reserve and flown on ordinary cargo and aeromedical missions from 1972 to 1982, frequently with the windows open because of the smell and the eye irritation. In 2010 the Air Force shredded the surviving airframes and melted the scrap into ingots.

Retired Major Wesley Carter filed an Inspector General complaint in 2011. After a January 2015 Institute of Medicine report, the Department of Veterans Affairs issued a rule effective 19 June 2015 presuming herbicide exposure for the roughly 1,500 to 2,100 aircrew, medical and maintenance personnel who had served aboard those aircraft. It took forty-three years.

The Vietnamese figures are larger and disputed. The Vietnamese government cites 4.8 million people exposed, 400,000 deaths or permanent injuries and 150,000 children born with severe birth defects; the Vietnam Red Cross puts up to a million people as disabled or ill. The United States government has described these estimates as unreliable. The veterans’ class action against the manufacturers settled in May 1984 for $180 million without going to trial.

In April 1975, President Ford formally renounced the first use of herbicides by the United States in any future war.

Sources: Buckingham, Operation Ranch Hand: The Air Force and Herbicides in Southeast Asia 1961–1971 (Office of Air Force History, 1982); Stellman et al., Nature 422 (2003); Lewis, “Smokey Bear in Vietnam”, Environmental History 11:3 (2006); Zumwalt & Zumwalt, The New York Times Magazine, 24 August 1986; U.S. Department of Veterans Affairs; ProPublica.