The appointment of Clifford L. Alexander Jr. as Secretary of the Army constitutes one of the most rapid promotions in American military history.

Back in the 1950s, about the time he was finishing at Yale Law School, Alexander served a six-month Army hitch at Ft. Dix, N.J. He was a reservist, one of the thousands of men who fulfilled the military obligation as a "weekend warrior." Alexander began and ended his military career as a private.

This will not, however, be Alexander's first experience with government at a high level, or with national defense. He worked in the White House and on the National Security Council during the period when the war in Vietnam was beginning to become a major concern of policy-makers.

Alexander, 43, has the mix of academic, corporate and social background that President Carter seems to favor in his new administration.

Born in Harlem in 1933, Alexander's parents were hard-working, middle-class blacks who concentrated their energies on their only child. Alexander's education, from preschool through senior high school, was entirely in private, predominantly while institutions in New York City, first the Ethical Culture School, then Fieldston.

Alexander went to college at Harvard and then to law school at Yale. In 1959, he married Adele Logan, whose father, Arthur, was a physician, friend, adviser and confident of black leaders and notables and the scion of a distinguished family that was part of America's black aristocracy.

After working for Manhattan District Attorney Frank Hogan for a stretch and then in some local neighborhood programs, including Harlem Youth Opportunities Unlimited, Alexander came to Washington in 1963 to work on the National Security Council staff.

Alexander worked under Michael Forrestal on the council, reading cable traffic concerning Asia, and especially Vietnam, to alert McGeorge Bundy, then head of the NSC, and through him President Johnson of anything unusual.

Forrestal has recalled Alexander as having "unusually high intelligence. He was able to pick up an entirely new field. He picked it up quickly."

In the summer of 1964, at the behest of Louis Martin, then deputy chairman of the Democratic National Committee, Alexander was moved from the NSC to the White House staff, as a deputy special assistant to the President. His assignment was to help Martin with legal problems associated with the Democratic National Convention, but Martin, a power in black Democratic circles, also wanted someone in the White House who could keep him accurately informed about official thinking in the field of civil rights.

It was during those White House days that Alexander first came into contact with Harry C. McPherson Jr., then one of the President Johnson's top aides and now one of Alexander's law partners.

"Johnson liked him, trusted him," McPherson said in an interview two years ago. "He had just been damn good, performed, had a lot of sense about government policy concerning blacks." Johnson rewarded him with an appointment to the chairmanship of the Equal Employment Opportunities Commission.

When Richard Nixon was elected President in 1968, it quickly became clear that Alexander was on a collision course with the new administration. Alexander quit before Nixon could fire him, criticizing the Nixon administration for a "crippling lack" of support for the fight against discrimination.

Alexander went to work at Arnold & Porter, one of Washington's largest and most influential law firms. Alexander, however, did not devote himself exclusively to law, he also served as the host for a television program, "Black on White."

While at Arnold & Porter, Alexander was a second-level partner - a "bonus" as opposed to a "percentage" partner - according to a knowledgeable source. Alexander's performance at Arnold & Porter was a source of some dispute.

"There were some people who did not think Cliff was pulling his oars and there were others who felt he was more than doing so," the souce said.

Alexander ran unsuccessfully for mayor of the District of Columbia in 1974. He made a respectable primary showing - 45 per cent of the vote - but never seriously threatened incumbent Mayor Walter Washington.

After the primary, Alexander went back to practicing law, representing corporate clients before the EEOC and other clients in other forums. In January, 1976, he left Arnold & Porter to join the firm of Verner, Liipfert, Bernard & McPherson. The firm added Alexander's name at the end and made him a partner.

Alexander, according to his wife, wanted "to be somewhere he felt he was part of the team making the deicisions and he wanted to be somewhere where he knew the other people."

McPherson, who served as deputy under secretary of the Army under President Kennedy, said the Army Secretary job "requires great political finesse" and an understanding "of the appropriate claims of the military on the society."

Alexander, McPherson said, would do the job well. "Cliff is a wise fellow. He has a personal ease and grace about him that will stand him in good stead and because those people over there are patriots."

Martin, who now is an executive with a chain of black papers in Chicago and the Midwest, continues to talk regularly to Alexander. Martin thinks the appointment is a good idea. "Cliff's got everthing - young, smart, objective," Martin said. You've got a guy here who knows both sides of the demic. He's a personable guy and this helpful. He had experience under LBJ, so he knows what to avoid, anyway.