A federal appeals court will soon hear oral arguments in lawsuits brought by citizens of Vietnam and American Vietnam War veterans who say their health has suffered from exposure to the defoliant Agent Orange.

The appeals are being brought under a variety of legal theories, and the court could arrive at opposite conclusions about whether the Vietnamese or American plaintiffs may go forward with their claims against the chemical companies that manufactured Agent Orange.

The arguments are set for June 18 before the U.S. 2nd Circuit Court of Appeals in Manhattan. They represent the latest round of Agent Orange litigation in a series of battles that have spanned nearly three decades. In 1984, several producers of the defoliant agreed to pay $180 million to settle a class action suit brought by American veterans. But that money has long since been paid out, and veterans who were not part of that suit say their symptoms only became apparent in recent years.

Agent Orange contains the most toxic form of the compound dioxin. The defendants, which include Dow Chemical Company, Monsanto Company, and Hercules Incorporated, dispute allegations that Agent Orange is the cause of cancers, birth defects, and other illnesses in the plaintiffs.

Named after the color of a stripe that appeared on its containers, Agent Orange was sprayed to chemically “pull back” the canopy of jungle under which the war was being fought. American forces sprayed Agent Orange over Vietnam for a decade, until America ended its defoliation program in 1971.

The 2nd Circuit judges are expected to hear more than three hours of arguments in the cases. The companies argue that the suit by the American veterans should be dismissed because the companies were government contractors when they produced Agent Orange.

The legal argument will shift for the case brought by Vietnamese plaintiffs, who include a former soldier in the North Vietnamese army and a former member of the Viet Cong. The chemical companies say this suit should be dismissed because Agent Orange should not be classified as a poison, the use of which in wartime is in violation of treaties, but as an herbicide.

“If it’s regarded as a chemical, it’s one thing, but if it’s regarded as an herbicide, that’s another,” a professor at Notre Dame Law School who in the 1980s served as the lead Justice Department lawyer in Agent Orange litigation, Jay Tidmarsh, said. “There is nothing wrong with using herbicide.”

No international law barred the wartime use of herbicides during the Vietnam conflict, the companies argue in their brief. This is a case about “how to conduct the war,” the brief argues, adding that this is a an issue for which “the judicial branch has not institutional competence.”

The lawyers note that the decision to use Agent Orange was sanctioned by three administrations and with the specific approval of Congress.

Lawyers for the Vietnamese will encourage the judges on the court to narrow their focus to the dioxin in Agent Orange.

“This is not a case about the use of a herbicide,” lawyers for the plaintiff write in their brief. “It is a case about the use of a poison.” Dioxin, the brief said, “was present in Agent Orange and other herbicides in excessive and avoidable amounts.”

Both suits were dismissed in U.S. District Court in Brooklyn.