FOR DECADES some people have been observing the lagging pace of change in South Africa and predicting an "explosion." Others have observed that the white minority's immense advantages in organization and armed power were bound to deter or, that failing, muffle any such confrontation. Both forecasts now seem to be coming true. A profound, almost prerevolutionary discontent is everywhere apparent; stirred by the familiar injustices of apartheid, it was brought to a boil by the black majority's evident collective decision that the government's reforms of last year were too little and too late. Meanwhile, the government is bringing its formidable powers of compulsion more openly to bear. The latest increment, in a society that already was for blacks a police state, is a declaration of emergency in the areas of Johannesburg and the eastern Cape.

South Africa is now undergoing the most serious unrest it has known. Strikes, statements, demonstrations, some anti-white sabotage and terror seem constant. Funerals, where blacks gather to mourn the victims of white guns, have become a major political venue. There is considerable violence of black against black but, far from being "mindless," it is plainly political, reflecting a strategy -- part spontaneous, part generated by the revolutionary African National Congress -- to destroy the limited forms of urban- black authority established by whites. Black protest, harassment, arson and murder have left only five of 38 local black councils operating; 240 black councilors, including 27 mayors, have resigned. The black police set up by whites to police black towns are under similar pressures. The point is unmistakable: the only legitimate black structures of authority are those that blacks create themselves.

Violence, of course, is the essence of apartheid: otherwise white power would vanish. Prime Minister P. W. Botha had undertaken limited reforms, although none touched the root problem of political power for blacks. Now he is responding to the consequent unrest with a turn of the screw. It treats the symptoms, in a way bound to breed greater alienation, and ignores the causes. Mr. Botha's defenders say he has no political mandate to move to political reform. In fact, he has no choice, if he is to halt his country's passage to a place of unending tension and strife.

Responding to this latest crisis, the United States indicated that unrest had justified the government's "new measures," and called for the unrest to abate so that the government could return to the path of reform. This is the pablum of "constructive engagement." The administration's inability to say that the people of South Africa are struggling for justice against a system that denies it to them could not have been demonstrated more clearly and more painfully.