The Western arms embargo on South Africa now in the making has come far too late to have any significant effect on that country's ability to wage conventional or guerrilla war against other African countries or its own black population in the forseeable future.

The embargo is particularly symbolic for the United States which is one of the rare Western nations to have followed fairly conscientiously the arms embargo imposed by the United Nations in 1963 and 1964.

The key nation in the success or failure of any new embargo now is France, by far the most important source of arms and technical expertise for South Africa and the main provider of licenses for its manufacture of such things as armoured cars and jets.

It is not much of an exaggeration to state that the South African government has been planning for this day for years.

While not altogether self-sufficient in the production of heavy weapons. South Africa already produces a wide variety of small arms and has developed with French help its own Cactus Crotale surface-to-air missle.

If the embargo does not cover foreign arms being manufactured under license in South Africa or contracts already made, it will have practically no effect at all.

Many of the most sophisticated arms in the South African arsenal - jets, armored cars and missiles - are produced with French, Italian and British cooperation in this manner.

Another gaping loophole could be the exemption of socalled "civilian" items , particularly in the aircraft field. South Africa for example, made extensive use of its commercial fleet of Boeing 747 Jumbo Jets during the civil war in Angola to ferry troops to air base in South African administered Nambia.

The United States, while by and large respecting the 1963 U.N. arms embargo, has sold South Africa six L-100 transports, the civilian version of the military C-130, and helped build a military communications and intelligence system for ship surveillance near Simonstown, South Africa.

For the last 10 years, the United States has also allowed the sale of about $27 million in spare parts for C-130 transport planes sold before 1963. President Carter indicated in his press conference yesterday that the United States might prohibit the sale of spare parts to South Africa.

South Africa has been methodically building its own military production capacity for over a decade. By 1965, it already had over 120 licenses for the manufacture of various foreign weapons and parts, including French armored cars. Belgian automatic rifles, Italian and French jet trainers and fighter.

The U.S. Arms Control and Disarmament Agency calculates that between 1965 and 1974, South Africa bought about $260 million worth of arms abroad with France alone supplying $225 million of this.

But the Stockholm International Peace Research Institute put the total value of all arms imports between 1963 and 1975 at slighty ever $1 billion.

French president Valerie Giscard d'Estiang has repeated on three separate occasions since he came to power that France would prohibit any further sale of ground and air equipment to South Africa, and also guarantee that no deliveries are made." A spokesman for the French embassy said yesterday no new contracts for ground and air weapons for South African forces have been made since 1974.

But U.S. officials said yesterday France has sold about $200 million in arms to South Africa that are still awaiting delivery.

France has also been deeply involved in the joint manufacture of arms inside South Africa. For instance, Panhard gave a license to that country in 1965 to build one thousand armored cars known locally as the Eland.

Another key French company has been the Societe Nationale Industrielle Aerospatiale and it susbsidiary. SUD Aviation, which was responsible for building South Africa's big Atlas Aircraft Corporation plant in the Travaal. There, Mirages and Impala jets are manufactured under license.

France also provides the bulk of South Africa's helicopters, Alouettes and Super Frelons, and various kinds of missiles and submarines. French firms are also building that country's first $1.2 billion nuclear power plant at Koeberg outside Cape Town.

South Africa also has on order an undisclosed number of Reshet gunboats equipped with Gabriel missiles from Israel.

Currently South Africa spends about $1.9 billion annually on defense - 18 per cent of its total budget and an increase of 21.3 per cent over lst year. Next year, it is expected to spend about $2.3 billion, a South African spokesman said yesterday.

South Africa's present level of expenditure on the purchase of foreign arms is not clear. The Stockholm Institute calculated it as $137 million in 1975, but the figure is known to have increased considerably in the past two years as a result of South Africa's participation in the Angolan civil war.

A recent article in the South African Rand Daily Mail reported that South Africa spent $930 million during the first half of this year on oil and arms together.But government figures group the two under one heading, making it difficult to know precisely how much goes for arms alone.

One estimate for arms purchases this year, based on the cost of imported oil last year, is between $200 and $300 million, but this sum could be slightly high. Nonetheless, the figure is substantial and has been growing rapidly for the past several years.

Compared to any of the black African countries on or near its borders. South Africa has a huge arsenal and a military expertise that is far superior to any of them now or in the near future. The Soviet Union is involved in arming Angola and Mozambique, but neither is likely to pose even a potential threat to the South Africans for years, and probably decades.

The most likely kind of warfare South Africa will have to deal with in the next few years is a guerrilla one - especially urban terrorism. But this kind of war is fought mainly with small arms an area where South Africa is totally self-sufficient.

So it seems that the far more effective measures against South Africa at this time would be such things as an embargo on loans and other economic steps. The economy is already in a recession and facing an 11 per cent inflation rate, so this sector is far more vulnerable than the country's military establishment.