Whatever legacy the premiership of Peter W. Botha leaves South Africa, a large part of the responsibility for it will rest with a reserved and studious man who is said to be Botha's closest adviser.

Gen. Magnus Andre de Merindol Malan is chief of one of the most powerful military forces on the African continent and only one that many believe has nuclear weapons capability.

Despite this potential superiority, Malan's speeches reveal a keen awareness of South Africa's long-term military vulnerability. For him, the threat lies not only in what Botha calls a total Marxist onslaught against his country but also in the internal rebellion that onslaught could cause if the majority of its people are discontented.

"It is clear that the world regards a physical trial of strength as the logical result of the friction in southern Africa," Malan said in 1976.

"We are even now committed to a war of low intensity, and a passive posture in the face of the assault must inevitably end in defeat," he said a year later. "This total struggle is one we dare not lose, because we will not be given a second chance."

When Malan was appointed by Botha in July 1976, he became at age 46 the youngest defense chief in South Africa's history.

Malan attended South Africa's equivalent of West Point, then in Pretoria, the town where he was born. His training also included a one-year staff course in 1962 at the U.S. Command and General Staff College at Ft. Levenworth, Kan.

Bypassing many senior officers, he rose rapidly and took command in 1966 of South Africa's fledgling counterinsurgency effort in Namibia (Southwest Africa). After a stint as head of the Military Academy he became chief of the Army in 1973, a post he held during South Africa's entry into the war in Angola.

Under Malan, the Army was expanded to include all races, and women have gotten more attention. His personal aide-de-camp, a captain, is a woman.

Despite his American experience, Malan clearly regards the West to be as much of a threat to South Africa as the Communist bloc. The war against South Frica comes from two sides, he told a newspaper in 1977. On the one band there is international communism with its friends, and on the other, the Western world and its humanitarian hangers-on.