With the dry season starting at the end of June, Jorge Sangumba emerged from the Angolan bush and crossed the border into Zaire to tell a story of yet another ignored opportunity to blunt the Soviet advance into southern Africa.

Sangumba is "foreign minister" of UNITA, the anti-communist losers in Angola's conventional war. After more than four months in the bush, he offered this report: UNITA's guerrilla operations have so dominated the countryside in southern Angola that Agostinho Neto's pro-communist government is endangered.

Objective outside sources confirm Sangumba's account of UNITA's success. A little arms aid from the United States or other Western sources indeed might topple Neto's Soviet-backed regime. The near certainty that there will be no such help underlines the fact that Soviet success in Africa is essentially a product of the West's weakened resolve.

Strategically and psychologically, Angola is vital to the Soviet campaign for domination of southern Africa. Angola is the staging area for guerrilla operations into South West Africa (Namibia), was the base for last spring's invasion of Zaire and is a training site for Rhodesian guerrillas. Beyond that, the U.S. withdrawal from Angola was noted by Africans as a sign of which super-power is the dependable ally.

Hence, Neto's present situation is bad news for the Kremlin. The unsuccessful coup attempt last month reflected black opposition to the ethnically separate Cabinda region in northwestern Angola, some 1,000 Cabindan guerrillas are much more troublesome (requiring Cuban troops to guard Gulf oil wells, Angola's principal source of foreign exchange).

But UNITA's operations over an area covering more than one-third of Angola are by far Neto's biggest worry. About 12,000 guerrillas, resupplied by discarded weapons, now control the countryside - mainly because of popularity with the black masses. Although UNITA holds no towns, some 5,300 UNITA delegates conducted a central committee meeting on the outskirts of Huambo (formerly Nova Lisboa), Angola's second city, early in May.

The 122-mm rockets (the fearsome "Stalin Organs") used by Cuban troops to panic UNITA forces during the civil war do not work against guerrilla operations. Indeed, Cubans are loath to go into the countryside against the guerrillas - partly because of a take-no-prisoners edict by UNITA. Sangumba's claim of 3,000 Cubans killed in guerrilla fighting is glven some credence.

As a result, the Soviet Union has been pressing Neto to bring UNITA leader Lucas Savimbi into a coalition government. But UNITA's central committee meeting decided against any such move so long as Soviet or Cuban forces remained in the country. "We know the Russians want to dominate," Sangumba told us.

UNITA is the kind of genuine nationalist movement that U.S. policymakers always hoped for in pre-Vietnam days. While avowedly socialist, UNITA is neither racist nor authoritarian. Its troop commanders include many native-born white Angolans of Portuguese extraction. UNITA pledges parliamentary democracy, and nobody seriously doubts that Savimbi would swamp all rivals in a free election.

Savimbi, in fact, is one of the continent's most widely respected leaders. While Rhodesian black nationalist leaders keep far from the bush and Holden Roberto stays in Kinshasa, Savimbi lives the life of an active guerrila chieftain. When the Cuban legions triumphed in Angola, Savimbi, instead of going into exile, returned to the bush where he had with foresight cached weapons.

With more weapons, UNITA could mount a force of 20,000 men. Sangumba left here for an extended tour through Western Europe and the United States, possibly in search of arms help. But even France, which kept Zaire from going under in the spring, is not likely to be helpful.

Without Cuban intervention, there is no doubt UNITA would have won the civil war. Even with Cuban intervention, South African troops fighting alongside UNITA would have reached the capital of Luanda had not the abrupt U.S. pullout made their position untenable. And now, a little arms aid could prove decisive.

But UNITA expects nothing of the kind, particularly not from Washington. "You Americans, we know, were traumatized by Vietnam." Sangumba told us (coincidentally echoing the phrase used to us by Zaire's President Mobutu), so, UNITA fights on alone for a democratic non-communist Angola, ignored by President Carter and Ambassador Young, who applaud the stability brought there by the Cubans.