Western fears of a major Soviet-Cuban involvement in the Rhodesian war presently seem vastly exaggerated and only justifiable if South Africa makes a major military commitment, including troops, to the new Muzorewa government in Salisbury.

Not only do the front-line states that back the Patriotic Front guerrillas seem not to want a large Soviet or Cuban presence in their countries, as Angola has, the guerrillas themselves do not believe outsiders should fight their war inside Rhodesia for them.

In any case, the Soviets and Cubans are bogged down in Ethiopia and Angola with no immediate prospect of extricating themselves from either of the wars that brought more than 30,000 Soviet-armed Cuban combat troops to Africa in the first place.

Separatists in Ethiopia's northern province of Eritrea and eastern Ogaden region have lost the big battles to the Ethiopians, thanks to massive Soviet and Cuban assistance. They have by no means given up their struggles, however, and continue to tie down 15,000 to 18,000 Cuban troops, mostly stationed in the Ogaden.

Dissidents in southern Angola under Jonas Savimbi, supplied and armed by South Africa, seem far from defeated and still keep the Benguela rail line closed. A series of Cuban-backed offensives by the government has failed to crush them and Cuba reportedly still has roughly 20,000 troops there, primarily in the south.

For the Soviets and Cubans, taking on the well-armed of Zimbabwe-Rhodesia would require a major commitment in arms and manpower, particularly if South Africa was backing the new government. Such an engagement would have enormous implications for their relations with United States and might well bring the Americans in.

Cuban President Fidel Castro has told guerrilla leader Robert Mugabe it is not Cuban policy to provide combat troops to liberation movements.

"He made it quite clear to me that they only support a state with . . . military units, when that state has appealed to them for that assistance," he told an American interviewer, Ruth Weiss, last summer.

Mugabe, like his rival partner, Joshua Nkomo, is now getting the help of Cuban instructors to train his guerrillas. Cuba does this for Mugabe in Ethiopia, where at least 1,000 of his guerrillas have now being trained, and for Nkomo primarily in Angola. The Cubans also do some specialized training back in Cuba.

South Africa, however, could upset the now carefully balanced equation that leaves outside involvement in the Rhodesian conflict at a generally low level.

Mugabe recently said in an interview with the French daily, Le Monde, that is South Africa openly intervenes in Zimbabwe-Rhodesia he first will ask African countries to send troops to help him. The inference seemed to be that he might eventually ask other countries, such as Cuba.

But what constitutes open intervention?

South Africa once had several thousand paramilitary police stationed and fighting in Rhodesia. It is reported to have some troops now along the Rhodesian-South African border that sometimes operate inside Rhodesia but are not yet permanently stationed there.

To date, there has been no suggestion of South Africa sending back large numbers of police or regular troops to Rhodesia.

South Africa already acts as the main conduit for all kinds of arms going ot Rhodesia from the West and sells it heavy weapons such as armored cars and, according to the guerrillas, even Mirage jet fighters.

Unconfirmed reports circulating in Salisbury say South Africa is ready to provide helicopters and their pilots, too, possibly as part of its stepped-up military assistance to the new government, which is seriously short of both.

Nevertheless, just what level of South African military involvement in Zimbabwe-Rhodesia will trigger a decision by the guerrillas or the frontline states to ask outside help remains unclear. CAPTION: Picture, Cubans troops, shown above in Angola with Angolan comrades, are believed unlikely to intervene in Rhodesia.AP