YES, THERE ARE within South Africa powerful and determined and even criminal forces conspiring to break down law and order, shred the fabric of the nation and undermine its international standing. They are the men in the government of Prime Minister John Vorster, who, intensifying repressive policy all too long in train, have just closed down three newspapers, including "The World," eloquent voice of the black community, outlawed some 18 black and white organizations working peaceably for social justice , added as many as 70 more people to the hundreds already "detained" or "banned" - seized or silenced by the state and denied the protection of the law - and promised more of the same "if necessary." Those "anarchists" whom the South African regime again accuses of fomenting trouble could not do the country as much harm in 10 years as Minister of Justice Jimmy Kruger, the man most behind the current moves, has done in one day.

Those moves will not, of course, curb the demands of the subject peoples of South Africa to be treated with dignity. They will, however, go far to ensure burial of the attempts at multiracial dialogue that moderate blacks have been making with special urgency since the Soweto uprising of 1976. These efforts offered South Africa the best, if not only, way to avert what Mr. Vorster himself, in a rare moment of insight, once called events "too ghastly to comtemplate." The moderates knew will how temuous their position was within the black townships.They will now be virtually helpless to refute the contention of their sons and daughters that violence is the only sure path to change.

There is, too, an international aspect to the latest triumphs of police rule. The whites, and especially the Afrikaners of the ruling National Party, are obsessed these days by fear that the United States is prepared to abandon, if not to destroy , them for geopolitical and political reasons of its own. Never mind that the paranoia is unjustified, or that it is cynically exaggerated and exploited by the leadership; it forms a central element in Afrikaner thinking all the same. It has produced a situation in which many whites will greet the new crackdown - conducted only hours after publication in South Africa of Vice President Mondale's calm and thoughtful appeal for dialogue - as a deserved rebuke to the United States for "interfering" in South Africa's internal affairs.

This unhappy political fact places upon Washington a delicate responsibility to make sure that its compassion for the victims of repression does not provide the practitioners of repression with the pretext for becoming even more arbitrary than they already are. Deciding how to deal with this dilemma is the somber central task now facing American policy on South Africa.