The government has at last come up with some hard evidence in support of its claim that chemical and biological warfare is being waged against defenseless peoples in Southeast Asia. The Soviet Union is almost certainly the responsible party, putting it in flagrant violation of a treaty commitment. Blood and urine samples taken from two victims of a chemical attack in Cambodia show the presence of one of the fungal poisons that the United States believes constitute "yellow rain."

Filling in troublesome gaps in previously available evidence, the new findings show that the victims have in their bodies sufficient amounts of a now identified poison to account for the severe reported symptoms. Control samples taken from individuals of similar age and background who were not subjected to the attack showed no evidence of the mycotoxin. Neither did samples of food, soil and water, making it extremely unlikely that the poison could have come from a natural source.

The new evidence is being sent by the U.S. government to the United Nations team that has been investigating the allegations of chemical and biological warfare. So far, the team has accomplished next to nothing. It reported in January that it was "unable to reach a final conclusion" on the accuracy of the charges. This was hardly surprising since the team had not managed to get itself into Laos or Cambodia or even to Afghan refugee camps in Pakistan. However, its mandate was extended, and the group is still at work.

So the ball is now squarely in the U.N.'s court. What is at stake is nothing less than the value of arms control treaties--if they can be flouted without punishment, are they worth the effort it takes to negotiate them?--and the rule of law. This is not a propaganda contest between the United States and the Soviet Union, but a matter that directly concerns the security of all nations, especially the developing nations. They are the likeliest victims of the development and use of these cheap, easily acquired, quiet--and lethal--weapons.