While Agent Orange may be the most well-known chemical used during the Vietnam War, it wasn’t the only one. An entire rainbow of new chemical formulations rained down on Vietnam’s forests and fields. The Rainbow Herbicides, as they were known, were only used as weapons in the war for a little over a decade, but their consequences can still be felt today.

The chemicals were deployed as part of Operation Ranch Hand, a military operation that lasted from 1962 to 1971. Ranch Hand’s unofficial motto—“only you can prevent a forest”—riffed off of Smokey Bear’s plea for people to prevent forest fires. The wry sarcasm of the phrase sums up the irony of the mission. Controversial then and now, it’s still not clear whether Operation Ranch Hand, a form of chemical warfare, was even permitted under international law.

Herbicidal warfare had been a military dream since the 1940s, when Allied researchers began to brainstorm ways to use chemicals to scorch the earth. However, early plans to use chemicals to, for example, starve the Japanese by ruining their rice crops, faltered.

In the 1950s, Britain became involved in the Malayan Emergency, an insurgency in a former British colony in what is now Malaysia. In an attempt to starve out Communist insurgents, British troops sprayed the lush forests with a substance similar to what became Agent Orange. The insurgents did fall, but the chemical spray had other lasting effects—severe soil erosion and lifelong health problems for Malayans.

“I remember the sight and the smell of the spray,” recalls Thomas Pilsch, who served as a forward air controller in South Vietnam in 1968 and 1969. In the early morning low angle sunlight, it appeared to have an orange hue.” By spraying Agent Orange, he thought he was helping the United States military bust through Vietnam’s impenetrable jungles on the way to victory.

The Geneva Protocol, developed after World War I to prohibit the use of chemical and biological weapons in war, would seem to forbid the use of these chemicals. But Britain argued that the conflict was an emergency, not a war—and that the treaty didn’t outlaw using chemicals for police actions.

The success of the operation—and its justification—prompted the United States to keep experimenting with the chemicals. In 1961, test runs began.

The U.S. had a rainbow of chemicals at their disposal. They were nicknamed according to the color on the barrels in which they were shipped. (Agent Orange didn’t appear orange, though it looked like that to Pilsch.) Once Operation Ranch Hand began, around 20 million gallons of Agents Green, Pink, Purple, Blue, White, Orange, Orange II, Orange III, and Super Orange were sprayed over South Vietnam. The chemicals were produced by companies like DOW Chemical, Monsanto, and Hercules, Inc.

“Trail dust” operations were conducted by the U.S. Air Force, whose “cowboys” flew C-123s escorted by fighters. As they approached a strategic target—dense, jungled areas that provided cover for the Viet Cong or crops suspected to feed their troops—the fighter jets would shoot down bombs and napalm. Then the sprayers would move in and douse an area with the chemical.

American soldiers were told the chemicals were safe. They were also effective. “We just blew away that jungle,” recalled Tom Essler, a U.S. Marine who served in Vietnam between 1967 and 1968, in an oral history. “Between the B-52 strikes and the Agent Orange, that lovely lush jungle around Khe Sanh was turned brown.”

As the jungle died, so did crops. Famine, malnourishment and starvation set in. By the end of the war, over 3.6 million acres had been sprayed with Rainbow Herbicides.

So had millions of Vietnamese people. (Though estimates vary, the government of Vietnam says that 4 million were exposed to the chemicals, 3 million of whom now suffer from health consequences.) American soldiers had also been exposed to the herbicides, reassured by their superiors that they presented no risk.

Not true: Sixty-five percent of the United States’ rainbow of chemicals contained dioxins—known carcinogens. Dioxins enter the bloodstream after being eaten or touched, build up in the food chain and can cause reproductive problems, cancer, hormonal interference, immune system damage, and developmental issues.

Contaminated soils, permanent forest loss, soil erosion, and other environmental damage have haunted Vietnam for years. It took years for the United States military to acknowledge that the chemicals were, in fact, harmful and even longer for them to begin compensating victims for their effects.

Meanwhile, the children of veterans and Vietnamese people exposed to the chemicals were born with serious birth defects and illnesses. In the United States alone, a ProPublica analysis suggests, a child born to a veteran exposed to Agent Orange was a third more likely to be born with a birth defect. And in Vietnam, people who lived beneath the rain of rainbow chemicals have experienced generations of health effects.

In recent years, it has become clear that not only did the government know about the herbicides’ awful effects, but that they relied on chemical companies for technical guidance instead of their own staff. The companies could have used fewer or no dioxins in their products, but they failed to do so. It’s an even more sobering twist to an already terrible story—one that keeps on illuminating the horrors of the Vietnam War decades after it came to an end.