THE United States is interested in what happens in South Africa. There are three reasons:
1. There are 26 million blacks in the US. They are increasingly active and influential politically. They care about what happens to blacks in South Africa.
2. South Africa is an important trading partner of the US. South Africa is the world's largest producer of gold, vanadium, aluminosilicate, chrome ore, diamonds, ferrochromium, and manganese. It produces 80 percent of the world's manganese. (The Soviet Union produces 14 percent.)
The US buys more from South Africa and sells more to South Africa than any other country. Britain is a close second in both directions of trade. West Germany is third. Japan is fourth.
3. South Africa is strategically important in East-West military rivalry. It is known in military parlance as one of the ``choke points'' in world strategy. Anyone who controls South Africa can control shipping around the Cape of Good Hope, and naval movements.
For these three reasons the US is interested in the future of South Africa and must hope for, and work toward, a future there that will be friendly to the US. At the very least, South Africa must not become a client and partner of the Soviet Union.
The problem for Washington is how to adjust its behavior toward South Africa in a manner that will avoid a gain for the Soviets.
Ideally (for the US) there would be a gradual and bloodless transition from white rule to shared black-white rule with a mixed government friendly to the United States.
There are ample lessons in recent history in how to conduct oneself in a situation like this. In China and Vietnam, the United States backed the ultimate losers. By backing the loser in China, the US delayed the break between Russia and China for 22 years. By backing the loser in Vietnam, the US bought itself the Vietnam war.
It is difficult for the US not to back the ultimate loser in civil war situations. Social and business relations have been built up with the old upper classes. The revolutionaries tend to be unknown in Washington. They didn't go to Yale or West Point. They are disturbers of the status quo and strangers.
Angola and Mozambique are also places where the US backed the losers. Moscow backed the winners. Moscow still has the inside track in Angola and Mozambique. Kenya and Rhodesia (now Zimbabwe) are places where the US backed the black winners against the minority white losers. Both are friendly and remain in the Western orbit.
In South Africa, there is an increasing tendency in the black community to see the US as supporting continued white rule.
The Reagan administration has attempted, or thinks it has attempted, to further the black cause through ``constructive engagement.'' The phrase is supposed to mean that the US urges the ruling whites to dismantle apartheid by private exhortations rather than by overt pressure. There may be substance to the theory, but in blacks' eyes the process has delayed, not speeded, the day of black emancipation.
Russia has so far played a minor and cautious role. The surprise is that it has done so little to encourage and arm the blacks in South Africa. But it has encouraged them, and it has probably supplied some weapons, directly or indirectly. Attacks on white targets by African National Congress guerrillas reached a record high of 100 incidents last year. So far the militant blacks have more reason to think of the Russians than of the Americans as their friends.
South Africa is singularly vulnerable to economic sanctions. It is ``heavily import-dependent.'' Imports require credit. There has been a credit squeeze by foreign banks, led by United States banks. The squeeze began in the spring of last year. The credit squeeze, plus black boycotts, general unrest, and black work stoppages, slowed economic growth in South Africa in 1985 to zero.
If South Africa's major trading partners, the United States, Britain, West Germany, and Japan, along with Switzerland, its No. 5 trading partner, agreed on a selective ban on trade with South Africa, it could undoubtedly force the white government to its economic knees.
A program of economic sanctions by the above five Western countries could probably win the friendship of the future black government of South Africa. Unless or until some such action is taken by Western governments, Moscow has a continuing opportunity to play the role of best friend to South African blacks and thereby gain the inside track in the black South Africa of the future.