A US federal court in New York has dismissed a legal action brought by Vietnamese plaintiffs over the use of Agent Orange during the Vietnam War.
The plaintiffs had sought compensation from the firms that manufactured the chemical, which allegedly caused birth defects, miscarriages and cancer.
They said use of the defoliant – to strip away forest cover during the war – was a war crime against millions.
But Judge Jack Weinstein ruled there was no legal basis for their claims.
The civil action was the first attempt by Vietnamese plaintiffs to claim compensation for the effects of Agent Orange, which has been linked to a multitude of heath problems, including diabetes.
However, the chemical companies said no such link had been proved.
The defendants – including Dow Chemical and the Monsanto Corporation – also argued that the US government was responsible for how the chemical was used, not the manufacturers.
They maintained that US courts could not punish corporations for carrying out the orders of a president exercising his powers as commander-in-chief.
In a 233-page ruling, Judge Weinstein threw out the case, saying: “There is no basis for any of the claims of plaintiffs under the domestic law of any nation or state or under any form of international law.”
The US justice department had urged the federal judge to dismiss the lawsuit.
The court decision in full (930K)
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In a brief filed in January, it said opening the courts to cases brought by former enemies would be a dangerous threat to presidential powers to wage war.
Between 1962 and 1971, large quantities of Agent Orange were sprayed across parts of Vietnam to deprive communist North Vietnamese forces of forest cover.
In 1984, several chemical companies paid $180m (£93m) to settle a lawsuit with US war veterans, who said that their health had been affected by exposure to the substance.
Agent Orange was named after the colour of its container. The active ingredient was a strain of dioxin that stripped the jungle bare.
In time, some contend, the dioxin spread to the food chain causing a proliferation of birth defects.
Some babies were born without eyes or arms, or were missing internal organs.
A group representing alleged Vietnamese victims says three million people were exposed to the chemical during the war, and at least one million suffer serious health problems today.