NBC News recently showed pictures of a pitched battle in South Africa between radical "comrades" and conservative vigilantes for control of Crossroads, a squalid slum in the Cape Town area. Even aside from the violence, the film was shocking. After weeks of government denials that it was provoking the bloody fighting, American TV viewers could see the vigilantes being escorted into Crossroads by the police. Then the killing began.

The vigilantes were armed -- many with machetes, some with pistols and a few with rifles. With such firepower, they carried the day, but the victory was a temporary one. With the passage of time, and the spilling of even more blood, the future almost certainly belongs to the radicals. Maybe when they take power, the United States will finally stir itself from its torpor and invoke economic sanctions.

In the meantime, the United States reacts to Pretoria's violent behavior with the sort of hollow condemnation we reserve for Israeli spying. When it comes to economic sanctions, the administration continues to say they would be counterproductive -- crippling the South African economy and, thus, ensuring the ultimate victory of radicals. What has not occurred to U.S. policy makers -- but what is readily apparent to South Africans -- is that the lack of such sanctions sends a message. South Africa's blacks are looking for an answer to that age-old question: Which side are you on? So far, our answer is that we are on everyone's side.

You need only spend a few minutes with someone such as the Rev. Allan Boesak, the anti-apartheid leader from Cape Town, to see how damaging this kind of response is to long-term U.S. interests. Ask him, as I recently did, what the image of the United States is in black South Africa, and you are told it "has never been as negative. You cannot mention the United States at a public meeting; you will be shouted down," Boesak said. America, he added, is considered Pretoria's pal -- more interested in keeping South Africa a member in good standing of the anti-Soviet alliance than in championing the human rights of the black majority.

In a recent speech, Secretary of State George Shultz said that, when it comes to South Africa, a concern for human rights must take precedence. But Shultz's voice does not carry into the black townships of South Africa. There, the personification of the United States is Ronald Reagan, and his values are well known. In his foreign policy, what takes precedence is containing the perceived Soviet threat, and to that end he will sacrifice almost all else.

Reagan, for instance, supports UNITA guerrillas in Angola, even though their patron in black Africa is white-ruled South Africa. In Central America, he mortgages our long-term interests to bloody a pipsqueak of a regime in Nicaragua. And in the Philippines, he delayed greeting the democratic government of Corazon Aquino, finally telephoning her and Ferdinand Marcos on the same day. Filipinos could be forgiven for thinking that Reagan reached Aquino because he thought Marcos still had the number.

The economic impact of South Africa sanctions is open to question. Maybe in the short run they will hurt poor blacks more than rich whites. But the idea, first and foremost, is to make a moral statement: to answer the question: "Which side are you on?" -- and, secondarily, to tighten the screws on the white government. As it is now, we are projecting the image of a banker -- calmly talking economic impact on the eve of a threatened race war. In Moscow, the Politburo must conclude geopolitical strategy sessions with a round of high-fives.

After all, it is the United States, and not the Soviet Union, that can boast of being a multiracial society. It is the United States that can boast of a civil rights revolution, of blacks in Congress, in the Cabinet and on the Supreme Court. The Soviet Union can make no such claim; yet it is the U.S.S.R. to which the "young comrades" look. Our most formidable weapon -- who we are as a people -- has been spiked.

But the president says little. His indignation resonates with less moral force than a veto message to Congress, and his voice was stilled entirely as television viewers watched the nightly killings. In the end, Reagan's fear of communism may turn out to be a self-fulfilling prophecy. We will get the South Africa we fear unless we help its oppressed blacks get the government they deserve.