Between 1962 and 1971, United States C-123 cargo planes sprayed 10 million gallons of the herbicide ''Agent Orange'' on the lush Vietnamese jungle. Bald patches left by the chemical are visible today from US photo satellites. The debate over its effects on US forces has proved similarly long-lasting.
Yet after a decade, there has been sudden progress towards solving the Agent Orange controversy.
The settlement of a mammoth Agent Orange lawsuit, announced Monday, ''is a big first step'' toward ending this intractable legacy of Vietnam, says Bart Stichman, litigation director of the Vietnam Veterans of America.
Still unsolved are scientific questions about the exact effect of Agent Orange on humans, and the difficult matter of what the US government owes those who feel exposure to Agent Orange has affected their health.
Agent Orange was not, in fact, orange. Its slightly insidious-sounding nickname came from the orange stripe around the 55-gallon drums it traveled in.
And it is not the chemical itself, in its pure state, that has engendered such lasting controversy. Rather, it is an impurity that batches of Agent Orange usually contained - dioxin - that has come under close scrutiny.
Dioxin is one of the most lethal substances known, analysts say. Thousands of veterans believe that exposure to the dioxin has caused a host of illnesses in themselves, and birth defects in their children.
In a class-action suit dating from the late '70s,these vets sued the manufacturers of Agent Orange - seven chemical companies, including Dow Chemical Corporation and Monsanto Corporation - for damages. Involving some 16,000 vets and with potential damages running into billions of dollars, it was perhaps the largest product-liability lawsuit ever filed.
But hours before jury selection was to begin May 7, lawyers for the vets and chemical companies reached a settlement. It establishes a $180 million fund to compensate veterans for health problems they claim are caused by Agent Orange.
The firms say the settlement was not an admission of guilt. Dow Chemical ''has not altered its belief that Agent Orange is not a plausible cause'' of vets' problems, says company spokesman Robert Charlton.
But a number of sources who have followed the issue say the large sum involved gives a stamp of legitimacy to the vets' complaints.
''In effect, the companies are admitting they're liable,'' says one congressional source.
Details of how the money will be dispensed must still be worked out. Some veteran groups worry that the funds won't be enough to go around. Bart Stichman of the Vietnam Vets of America figures it allots about $9,000 per individual covered by the class-action lawsuit.
''Is the amount going to be adequate?'' he says. ''We don't know yet.''
And there is still a long way to go before the issue of Agent Orange is completely settled.
Despite thousands of studies of dioxin, scientists say, we don't yet know exactly how it affects humans. The lawsuit, even if carried to its conclusion, wouldn't have been able to establish more than a circumstantial link between Agent Orange and veterans' maladies.
''I'm not convinced by either those who say dioxin is the sole cause of their health problems, or by those who say it isn't the cause at all. The right experiments just haven't yet been done,'' says Harvard biochemist Matthew Meselson.
Establishing the effect of Agent Orange is made difficult by the fact that US forces in Vietnam were exposed to a whole host of dangerous substances. The wide range of illnesses vets lay to Agent Orange further complicates the investigation.
The most wide-ranging study yet done, an Air Force effort released in February, was inconclusive. Research sponsored by the Veterans Administration won't be finished until the end of the decade.
Also unsettled is whether the US government should set up its own mechanism for compensating veterans who were exposed to Agent Orange. On Jan. 30, the House passed a bill that would compensate all Vietnam veterans suffering from any of three maladies vets claim are caused by dioxin. The Congressional Budget Office predicts the bill would cost $25 million over five years.
The Senate may vote on a similar bill this summer. Another approach, advocated by Sen. Alan Simpson (R) of Wyoming, would consider claims on a case-by-case basis.