Attorneys representing major U.S. chemical companies defended them against charges on Monday that the companies committed war crimes by supplying the military with Agent Orange during the Vietnam War.

The lawyers asked a U.S. District Court judge in Brooklyn, New York, to dismiss a civil suit that seeks class action status claiming that up to 4 million Vietnamese people suffered from dioxin poisoning due to Agent Orange.

The case is regarded as a pivotal test of the reach of U.S. courts as it considers the power of the U.S. president to authorize use of hazardous materials during war.

More than 30 companies are named in the lawsuit, among them Dow Chemical Co. and Monsanto Co.

If the lawsuit were successful, billions of dollars could be awarded toward an environmental cleanup and in compensation to the Vietnamese people.

Judge Jack Weinstein is expected to issue a written decision in the next few weeks.

The defoliant Agent Orange was dumped by U.S. warplanes on Vietnamese forests between 1962 and 1971 to destroy Vietnamese sources of food and cover.

Among the chemical by-products of Agent Orange is dioxin, a compound that can cause cancer, deformities and organ dysfunction.

The chemical companies argue they produced Agent Orange according to U.S. government specifications and that there has never been a proven connection between Agent Orange and the health problems it is accused of causing.

Outside the courtroom, Andrew Frey, an attorney for Dow, said the issue should be decided by “diplomatic negotiations” and not by the lawsuit.

“We think it is up to the United States government to decide whether what it did was wrongful and whether it should pay restitution,” he said.

He added that international laws in the 1960s did not recognize corporate liability and the courts should be cautious about ruling on cases affecting the president’s power.

“The court should not be second-guessing the president’s decisions, which were made after studying the human health consequences and as a military judgment and very likely saved a lot more lives than it injured,” he said.

One of the plaintiffs, Dr Phan Thi Phi Phi, said through a translator she worked in an area that was heavily sprayed with Agent Orange and suffered four miscarriages over two years during the early 1970s.

She said the effect had been “devastating” and that she knew of many other cases like her own.

“We did not know what happened to us, what was the cause of it, so we were very sad because we had so many miscarriages and we could not have children,” she said.

Her attorney Constantine Kokkoris argued that people in Vietnam continued to be contaminated by eating tainted food and drinking tainted water.

Lawyers for the plaintiffs have cited precedent from the years following World War Two, when makers of the gas used in Nazi death camps were convicted of war crimes.

In 1984 seven chemical companies, including Dow and Monsanto, agreed to settle out of court for $180 million to U.S veterans who claimed Agent Orange caused cancer and other health problems.

Dave Cline, President of the Veterans for Peace, said it was important the Vietnamese people were treated the same way as U.S. veterans.

“We have been able to get American Veterans recognized and now it is time to give Vietnamese victims the same justice,” he said.

Other companies named in the lawsuit include Hercules Inc., Occidental Chemical Corp., Ultramar Diamond Shamrock Corp., Maxus Energy Corp., Uniroyal Inc. and Wyeth.