The United Nations, which is run largely by and for the benefit of tyrannical regimes, is divesting its pension fund of South African assets. Various European governments that are purchasing gas pumped through a Soviet pipeline built by slave labor are suddenly stern about South Africa. And the music of American moralism has reached fortissimo regarding South Africa, with a brisk staccato of demands for disinvestment and other gestures involving no noticeable risk or even inconvenience for those doing the demanding.

Clearly some of the current campaigning against South Africa is a fad, a moral Hula Hoop, fun for a while. Regarding interest in a foreign crisis, even altruism is not fuel for the long haul. For a foreign crisis to preoccupy a relatively content society such as ours, it must affect a vital interest of a majority on a continuing basis. Injustice in Africa does not. Not even the very vocal spokesmen for black Americans are audible often about the tyranny of black despots over the majority of the 400 million black Africans.

The New York Times recently carried this melancholy headline: "Uganda Regains / Uneasy Normalcy." In South Africa in the last year, more than 600 persons have died in political violence, some of them blacks and Indians killed by blacks. Six hundred has been the average weekly death toll during the last decade of Ugandan normalcy.

Nevertheless, the manifest and manifold injustices of South Africa's system make economic sanctions a temptation, because they can make us feel good. But should they?

Right-mindedness is not right behavior. If the aim of sanctions is described modestly enough, the success of sanctions is ensured. That is, if the aim is to express disapproval, sanctions cannot fail. Sen. Richard Lugar (R- Ind.), chairman of the Foreign Relations Committee, says sanctions would say "how we feel" about apartheid. But American opinion already is clear -- indeed, almost unanimous -- in disapproving apartheid.

President Carter's grain embargo and Olympics boycott in response to the Soviet invasion of Afghanistan were useful. They were useful not because they could do serious damage to the Soviet Union, still less because they would alter Soviet behavior, but because the measures helped awaken Americans from slumbers that Carter, among others, had induced with talk about "inordinate" fear of communism. Economic pressure against the Soviet empire should be continuous because it is our mortal enemy and its militarism should be burdened as much as possible.

But what of South Africa? Are sanctions supposed to destabilize it? One reason there is a Reagan administration is that the preceding administration helped bring down the shah and Somoza, two exercises in making matters worse. Are sanctions to remain in place until Pretoria changes policies? If so, which policies?

Simon Jenkins of The Economist, writing in The Spectator, notes that force, not the sanctions, settled disputes involving Rhodesia and Argentina. The Arab oil embargo did not erode U.S. support for Israel; it stimulated conservation and development of alternative energy sources, which have weakened Arab economies.

Thanks to an oil embargo against South Africa, it is nearly self-sufficient, with the world's best process for producing oil from coal. Thanks to an arms embargo, South Africa, which was 60 percent dependent on imported arms 20 years ago, today is 90 percent self-sufficient and a net arms exporter.

Sanctions would raise the costs of apartheid, but as Jenkins says, "Apartheid is not a white tribal hobby to be dropped from the household budget when things get tight." Sanctions, he says, please people who believe they are entitled to reorder the world and that the reordering can be done without violence: "The modern crusader sits at home ripping up IBM stock. . . . Defeat is someone else's fault and only the poor get hurt." In southern Africa, the 40 million poor would include many millions in states that are, and will remain, linked economically with South Africa.

Before the American Civil War, some abolitionists considered dissolution of the Union -- secession by the North -- preferable to continued association with slave states. This policy would have left the slaves to their fate, but their fate was not the most important thing to those abolitionists. The most important thing was the abolitionists' self-regarding fastidiousness about their own moral hygiene. Some advocates of sanctions and other measlate South Africa seem most eager to isolate themselves from what is apt to be a long, tedious, morally ambiguous and largely unsatisfying process of constructive pressure through continued engagement.