The United States, it was revealed over the weekend, has begun a major review of its policy toward South Africa. Good.

If the first casualty of that review is the peculiar blend of naivete and cynicism known as "constructive engagement," better yet.

For virtually the life of the Reagan administration, American policy has sought, by dangling the carrot of warm diplomatic relations, to lure Pretoria in the direction of change -- if not thoroughgoing reform, at least enough reform to give international legitimacy to the white-minority government.

At first, the administration might really have believed that U.S. good will could coax the South Africans into significant change, and that official hostility -- not just sanctions but even harsh words -- would only drive the Afrikaners into their wagons against all outsiders.

When this belief turned out to be hopelessly naive, the administration switched to cynicism, painting its velvet glove to look like like iron. Whatever misgivings President Reagan might have harbored about the policy of apartheid were outweighed by his wish to maintain South Africa as a Western-style business partner, a source of strategic minerals and a bastion against communism. Thus, Reagan's recent approach had been to talk tough while giving the South Africans time to work out an acceptable solultion.

The weekend's disclosure of a high-level policy review may be an acknowledgment that cyncism didn't work either.

"I think the South African government is becoming more rigid, and that's unfortunate," a senior official said Saturday, adding that Pretoria "is beginning to develop the attitude that they don't particularly care what the rest of the world thinks. They are going to maintain their control and try to reduce the violence that way. I don't think they can succeed indefinitely. . . . The disparity in the number of people is just too great."

The Reagan administration might not have succumbed to justice, but it at least has discovered arithmetic.

It is dangerous to try to read too much into what is, after all, not a new policy but merely a decision to rethink the old policy. But the administration seems to have concluded that profound change will come to South Africa, no matter what Washington does, and,that being the case, that it might be a good thing at least to be on speaking terms with the leadership of the nonwhite majority.

That's a smart conclusion, in terms of both justice and the American interests. The United States is increasingly viewed by South Africa's franchiseless majority as an ally of its oppressors. We have bought the Pretoria line that the white government is a stronghold against communism while the African National Congress, which clearly has the support of most of South Africa's blacks, is little more than a commie front. U.S. officials refused to see the ANC's exiled leaders, President Oliver Tambo when he visited Washington last April, and Secretary-General Alfred Nzo, who was here earlier this month.

That will be changing, administration spokesmen say, noting that some "low-level contacts" with ANC officials already have been established.

That's a good beginning. Until now, the administration seems to have taken the view that no good solution to South Africa's crisis is possible, and that to put serious pressure on Pretoria to make significant concessions is to urge the white minority to suicide.

But now Washington seems to recognize that the surest way to white suicide is to refuse to undertake serious change.

All that remains now is for the Reagan administration to understand that it needn't attempt to prescribe solutions but merely provide the pressure necessary for South Africans to work out their own solutions.

In a word: negotiations.

The best role for the United States now is to use the leverage at its disposal -- whether sanctions or other pressures is of no particular importance -- to make sure that the real representatives of the black majority -- including the banned ANC and its jailed and exiled leaders -- are parties to the negotiations.

There is still time to work out arrangements for power sharing that will preserve a role for whites. But not much time. The sooner the process begins, the better the chances for avoiding an increasingly likely bloodbath.

"Constructive engagement" may be dead, but the oppportunity for the United States to play a constructive role in South Africa isn't. At least not quite yet.