BARELY A YEAR AGO South Africa agreed, in a deal brokered by the United States, to remove all its raiding forces from Angola in return for Angola's closing down the bases from which SWAPO guerrillas operated in Namibia. Now, with some of its forces still in Angola, it is revealed to have sent a sabotage squad hundreds of miles farther north to attack an American-run oil installation in Cabinda province. The squad, carrying incendiary shells, was intercepted 300 yards from the Gulf facility that earns Angola most of its foreign currency. An Angolan mission was reported in New York on that very day seeking expansion capital: evidently the South Africans meant to spoil Angola's day in a spectacular way.

South Africa's policy is illuminated as by a midnight flare. It pummeled Angola into an agreement that much diminished the guerrilla challenge Pretoria faced in its illegal colony of Namibia, which, in the fifth year of the Reagan administration, seems still remote from independence. The same agreement left the Angolan government facing a strong South African-fortified guerrilla challenge of its own. And having made the agreement, Pretoria kept on selectively mounting the same destabilization tactics it had promised to abandon.

The United States is left looking very foolish. A friendly government for which it did a great favor has duped it. Let us try to imagine the anti-communist nonsense that Pretoria would have pumped out if its commandos had secretly succeeded in blowing up Gulf Oil. The Reagan policy of "constructive engagement" -- treating South Africa in a respectful fashion to gain its moderation in foreign policy and domestic policy alike -- is embarrassed on the very ground where its greatest triumph had been proclaimed. Angola's Marxist government, meanwhile, hasw excuse to embrace the Cuban protectors whose removal is the administration's prime regional goal.

We were among those who a year ago felt cautiously hopeful about the prospects of American diplomacy in southern Africa. But things have not been going well. The other principal front where South Africa's policy of regional accommodation has been on display is in Mozambique. There rebels continue to assault the local government even as South African authorities insist that they do not sponsor and cannot control the foreign elements that help keep the resistance going.

This is the other face of apartheid. The system not only represses the black majority inside the country. It also gives the white minority in the leadership the power and the arrogance to torment South Africa's neighbors. The people of the region need the abolition of apartheid scarcely less than do the people of South Africa.