Richard L. Walker, the U.S. ambassador to South Korea, has yet another explanation for how it was that a squad of security police roughed up a group of Americans accompanying opposition leader Kim Dae Jung home to Seoul. The Americans with Kim said the Koreans started the melee. The Korean government said it didn't start anything. But the ambassador has finally offered an explanation that makes sense: He's blamed Patt Derian.

Actually, Walker blamed the entire group of Americans who had accompanied Kim. But Derian is not only probably the best-known member of that delegation but also precisely the sort of person who would step between the police and an opposition leader whom Korean authorities have tried to kill in the past. She's been doing that sort of thing since the civil rights days in Mississippi.

Today Derian's passion is civil rights writ global, which explains her presence at the airport. She and the other Americans were determined that what happened to Benigno Aquino, the Philippine opposition leader who was gunned down in Manila, would not happen to Kim. If anything, it was the very assassination of Aquino that made a repeat, Korean style, unlikely. But the death of one dissident in Manila is no guarantee that another would be allowed to remain alive in Seoul.

So what is ambassador Walker talking about? Who cares if Derian and the other Americans accompanying Kim broke their agreement and refused to allow the Korean police to take Kim off the plane by himself? They insist they made no such agreement and that they did absolutely nothing to provoke the police (yet another Walker charge). But none of that really matters anyway.

The fact remains that Kim is under house arrest. He cannot even go to church, and ministers who have come to see have been turned away by the police. He was once kidnapped in Japan, probably by the Korean CIA, threatened with death and dumped back in Korea. He's been imprisoned, exiled and attempts have been made on his life.

There is but one other fact you should know about Kim: his crime. There is none, unless it is near-success as an opposition politician. In 1971, running as the opposition candidate for president, he received more than 45 percent of the vote, which, to provide some perspective, is better than Walter Mondale did against Ronald Reagan. Mondale, though, went back to his law firm. Kim was jailed for the crime of dissent.

Maybe in the crush the police lost their cool. Maybe the government really had intended for the cops to be well-behaved. And maybe, even, Derian and the other Americans panicked at the last moment and refused to let Kim out of their sight. If they did, they had their reasons. They, if not the ambassador, knew that it was the police who threatened Kim -- not the other way around.

The other way around, though, is apparently the way the ambassador and the Reagan administration prefer it. They have directed their outrage at Derian and the others. For the government that over the years has imprisoned Kim, kidnapped him, tried to kill him, exiled him and now has him under house arrest for the crime of political opposition, they have uttered only the mildest rebuke. The president himself blamed the melee on "bad judgment on both sides." This, like the "constructive engagement" of U.S. South African policy, is foreign policy without a soul -- moral vacuity posing as Realpolitik.

It hardly matters who, if anyone, broke an airport agreement. What matters is that an ally, a country whose independence was secured by American blood, roughed up two congressmen and two former American diplomats who only wanted to protect the life of a political dissident. The ambassador is right: Derian is a perfect example f an American who cannot be trusted. At any moment, she's likely to do the right thing.