Southern Africa is calm because all know who's boss.

For nine years, white-ruled South Africa has been largely reacting - often aggressively - to a growing black regional and internal challenge. But today, South Africa appears to be regaining the political momentum in southern Africa and for the time being seems to have seized the initiative from its challengers.

This development could have important implications regarding the disposition and longevity of white power in South Africa, say close observers of the region. It is already seen as putting immense pressure on the forces of black nationalism opposing the Pretoria government to reconsider their strategies.

For the moment, South Africa is using its commanding position to impose a kind of peace in the region.

The most dramatic developments include peace agreements between South Africa and its two archenemies of the subcontinent, Angola and Mozambique. South Africa and Mozambique have agreed to sign a formal nonaggression agreement - the first ever between South Africa and any of its neighbors.

And in southern Angola, this country and South Africa have agreed to a cease-fire and ''disengagement'' of South African forces. A joint commission began monitoring the process on March 1. The breaking out of peace in southern Angola is generating new hope about a settlement to the protracted dispute over Namibia (South-West Africa).

All of this is welcome in a region that was rocked by violence on a number of fronts less than a year ago.

But the peace comes largely on South Africa's terms, say analysts. ''The neighboring states were battered to the point where they could not go on,'' says Hermann Giliomee, a political scientist at the University of Cape Town.

Pretoria has essentially demanded that its neighboring black states remain neutral in a strategic sense as South African blacks continue their drive to gain meaningful political power at home, analysts say.

Some analysts wonder how long peace on these terms can last. They wonder whether the black states of the region will be able or willing to remain neutral in the long term unless Pretoria broadens its vision of internal ''reform'' and begins to address fundamental black grievances. South Africa appears to have regained the initiative both internally and regionally with a strategy largely put in place by Prime Minister Pieter W. Botha, a former minister of defense.

Under Botha, South Africa has taken the war to its enemies, with an aggressive military posture that has hurt neighboring states militarily and economically, say close analysts here.

While asserting South Africa's military and economic dominance, Botha has introduced a policy of so-called ''reform'' within South Africa.

By and large, this ''reform'' has not addressed fundamental black grievances, most analysts agree. But the policy has smoothed the rough edges of apartheid, making it what some analysts call more ''rational and coherent.''

The unanswered question is whether so-called ''reform'' is meant to simply make apartheid more sustainable, or whether it may prove to be the precursor to changes at the core of Pretoria's policy of excluding the black majority from meaningful political power.

Here is a closer look at how South Africa has regained the initiative both inside and outside its borders:

* Internally, Pretoria's white rulers appear to be concentrating power in the hands of a small ruling elite with a slightly more pragmatic outlook, while at the same time broadening the base of their political support.

A proposed new constitution, which many analysts say furthers both of these aims, is expected to be implemented later this year.

The constitution would bring Coloreds (persons of mixed-race descent) and Indians into the previously all-white Parliament as junior partners. Elections for these groups will be held in August. The government has left blacks out of this new arrangement.

The broadened Parliament will earn the government some new nonwhite political allies, analysts say. The proposed constitution also appears to have broadened the government's base of white support. In a white referendum on the constitution last year, a large number of English-speakers supported the ruling Afrikaners.

The new constitution also establishes a more powerful presidency for South Africa. This is in keeping with a more centralized and streamlined decisionmaking process already introduced by Prime Minister Botha, analysts say.

Many close observers believe key decisions in South Africa are no longer made by the Cabinet but by the military-minded State Security Council. The rise of the State Security Council parallels a general militarization of South African society under Botha, some analysts say.

Robert I. Rotberg, a political scientist at the Massachusetts Institute of Technology, has done an extensive analysis of Botha's style of governing. He concludes there has been a shift in the locus of power from old-line bureaucrats to a ''cadre largely composed of military-trained technocrats.'' Rotberg says Botha's aim is not to ''transform South Africa,'' but to make it more tactically sound. These important political developments are taking place with little or no consultation with blacks, who remain excluded from the new constitution. The government continues to proclaim that blacks are following a ''different constitutional path,'' referring to its policy that blacks exercise their political rights in the remote and generally resource-poor ''homelands.'' However, the government has acknowledged that ''new structures'' must be considered for blacks living in ''white'' urban areas outside the homelands. A committee of Cabinet ministers is studying how to accommodate these blacks politically.

Black political opposition in South Africa has not disappeared. Indeed, Pretoria's new constitution appears to have hardened black resistance.

But black opposition remains divided and faced with the difficult task of finding avenues of political dissent that are both legal and effective, say analysts of black politics. South Africa's security laws are highly restrictive. And even when new black political movements are not banned, they are harassed by the state.

* Regionally, South Africa is reasserting its influence and dominance after years of being on the defensive.

Pretoria has amply demonstrated its military hegemony in southern Africa since about 1981, hammering neighboring states that supported the rising insurgency against South Africa.

The platform once provided by these neighboring black states for the outlawed African National Congress - the main challenger to Pretoria - appears to be crumbling beneath the ANC's feet.

South Africa also appears to have the initiative on the economic front. Pretoria has demonstrated it has enormous economic leverage over its neighboring states. And with all of southern Africa suffering from recession and drought, these neighboring black states are in no position to allow their economic dependence on South Africa to be upset further, say economic experts.

The Southern African Development Coordination Conference was established by the black states of the region in 1980 to help them extricate themselves from Pretoria's economic embrace and the political leverage it provided. Despite gains, SADCC appears to have adopted a more accommodating posture regarding South Africa, say analysts. Indeed, at a meeting last month, SADCC said it welcomed South Africa's ''less aggressive stance'' in the region.

One central question is how long Pretoria can retain the initiative against the basic antipathy of the black states of the region and of it own black majority.

Clearly, this antipathy is not going to evaporate, say close observers of this region. But Pretoria appears to have developed a coherent strategy for denying black opposition any foothold either internally or regionally from which to build momentum in the foreseeable future against the white-dominated government.

There are basically two schools of thought on the implications of an ascendant South Africa.

Some analysts favor a rather optimistic scenario. The way they see it, South Africa's white rulers are faced with a gathering domestic political crisis in the form of growing black demands for meaningful political power. These analysts believe Pretoria recognizes this and is likely to be bolder with meaningful reforms involving blacks if it is confident and feels more secure. A new executive president may also be inclined to act more boldly, some argue.

This argument also envisions South Africa playing a more constructive role regionally once it has neutralized in a strategic sense the black governments on its borders. Some believe the most dramatic ''concession'' by South Africa may be its eventual final surrender of control over Namibia.

Other anlysts paint a more pessimistic picture. In their view, an ascendant South Africa is an intransigent South Africa. These analysts believe Botha's overriding aim is to make apartheid irreversible.

Most analysts do not believe South Africa was ever seriously threatened by the cascade of events that transformed southern Africa beginning in 1975.

But the changes in the region were swift, dramatic and profoundly disturbing to South Africa's white rulers. Close observers of that period believe Pretoria was principally reacting to events that the government eventually called a ''total onslaught.''

The year 1975 was a watershed for southern Africa. In that year, Portuguese rule in Angola and Mozambique came to an end, replaced by Marxist black leaders who had successfully waged wars of ''liberation'' against their colonial rulers.

The development caused South Africa regional and internal concerns. Black expectations about ''liberation'' in South Africa were raised dramatically. At the same time, Pretoria portrayed the new black governments of Angola and Mozambique as part of a Soviet pincer movement aimed eventually at gaining control of South Africa. This heightened whites' ''threat perception'' and generated strong support for the growing influence of the South African military.

The coming of black governments in Angola and Mozambique dramatically improved the position of the ANC. The ANC was banned in South Africa in 1960 and attempted an underground sabotage campaign beginning in 1961. But the campaign was largely unsuccessful from the mid-1960s to the mid-1970s.

The new independent government of Angola allowed the ANC to establish military training bases in that country. And Mozambique provided access for ANC guerrillas to enter South Africa. This allowed for the beginning of a meaningful insurgency in South Africa, experts say.

This regional upheaval was followed shortly by South Africa's most serious internal unrest. Black student unrest in Soweto in 1976 quickly accelerated to a widespread black revolt throughout South Africa. It was the most serious black challenge Pretoria had ever faced, analysts say.

While the state crushed the Soweto revolt, it led to a sizable exodus of blacks, many of whom joined forces with the outlawed ANC.

The next major development was Zimbabwe's independence in 1980. South Africa was again put on the defensive when the moderate black candidate it most favored was soundly defeated by Robert Mugabe, a staunch opponent of South Africa.

Mugabe's victory, along with a dramatic rise in insurgency, ''brought South Africa into confrontation with our neighbors and put us on the defensive internationally,'' says one political analyst here.

In 1981 South Africa began to lash out with military strikes and apparent increases in aid or support to rebel movements of neighboring states. It raised the stakes of the long-simmering Namibian dispute by penetrating deep into Angola in 1981 and then permanently occupying parts of that country. That occupation is only now coming to an end under the ''disengagement'' agreement between South Africa and Angola.

Next: Black states' shifting strategiesm