NAMIBIA is back in the diplomatic news. A colony of South Africa, it should have gained independence decades ago. The General Assembly pushed the issue for years in vain. Jimmy Carter then arranged to have the United States, Britain, France, Germany and Canada take over the project, under Security Council aegis. He, too, fell short, for one reason: because the South Africans felt they might get a better deal from Ronald Reagan. So far, in a kind of self-defeating way, they have.

What happened is that Mr. Reagan put a new demand on the table: the Cubans in Angola must go home. This was risky. Everyone sees the natural linkages: if Namibia were independent the SWAPO guerrillas wouldn't need sanctuaries in Angola, and without those sanctuaries South Africa would not run attacks into Angola, and if Angola were not threatened it could send its Cuban militia home, and if there were no war across the border, the South African-supported insurgency in southern Angola could more easily be diverted to political channels. But these are connections best kept fluid and informal. The administration hardened them up.

The result is the frustration of all three of the administration's goals in southern Africa. The Cubans remain in Angola. Namibia is still denied its independence. Nor has there been a detectable return, either a softening in Namibia or an easing of apartheid, on the policy of "constructive engagement" (improved relations) with South Africa itself.

The requirement now, it seems to us, is to heed the other four of the five Western negotiators and to loosen up the Namibia-Angola linkage. South Africa will remain reluctant to yield power in Namibia to SWAPO, the main guerrilla force and the almost certain winner in any elections. But perhaps some flexibility can be imparted to the talks. There appears to be some factional play among Angolans on the Cuban presence -- enough in any event to give American diplomats some hope that it's worth staying at the Namibia table.

As a practical matter, it is said, the South Africans won't free Namibia until the Cubans leave: the Botha government, hard-pressed anyway, might topple. But also as a practical matter, the American focus on the Cubans lets South Africa slip the difficult choices confronting it. As desirable as it would be for the Cubans to leave Angola, it is impractical -- and unfair -- to try to make the independence of Namibia formally contingent upon that happening now.