Alexander M. Haig Jr., the 56-year-old former general nominated yesterday as the next secretary of state, has sat near the center of power in this country as long as any other public figure -- military or civilian, Republican or Democrat -- in the past 20 years.

By any measure, Haig has had an extraordinary career, beginning as a bright young aide on the staff of Douglas MacArthur in Japan in 1949 and then spiraling steadily and rapidly upward in the service of Cyrus R. Vance, Robert S. McNamara, Henry A. Kissinger, Richard M. Nixon, Gerald R. Ford and Jimmy Carter.

Now, a fourth American president, Ronald Reagan, is placing the biggest and most controversial bet ever made on the qualities that have catapulted Haig from the rank of colonel in 1969 to four-star general in 1972 to would-be secretary of state in 1981.

Reagan's bet on Haig is two-fold: that the former White House chief of staff in the final stage of the Nixon administration can be confirmed by the Senate without any embarrassing new information about the Watergate era exploding and thus tainting the new administration, and that a man who spent 35 years in the military has the diplomatic skill and sense of the broad range of American interests to manage U.S. foreign policy during a period of global change and unrest.

When Haig -- to the dismary of many military colleagues -- was zooming through the Army's ranks in the early 1970s, with the help of Kissinger and Nixon, Army officers said at the time they could think of only two parallels for such a meteoric rise. One was George A. Custer, who eventually was killed by Indians in the Battle of Little Big Horn in 1876, and the other was George C. Marshall, who went on to serve as President Truman's secretary of state from 1947 to 1949.

If confirmed, Haig will become only the second military man to hold the secretary of state job.

Haig's general views on where U.S. foreign policy should be heading are well known. He has called for a "reassertion of American leadership" in which Washington must lead first by dealing effectively with the nation's economic and energy problems while showing a willingness to match the Soviet military build-up and to "counter Soviet interventionism." Otherwise, he warns, U.S. "pretensions to leadership will be derided."

Rather than resort to "bullying and insensitivity," the former NATO commander has said, the United States must get its allies to make the tough decisions for common defense and policy by an ability to "inspire, persuade, urge and cajole." He sees advantages for the West in close ties to China and danger in the idea that West Germany might someday find it attractive to become neutral, perhaps because Washington mishandled relations with Bonn.

Haig's ability to carry out such a policy, however, could go beyond the secretary's traditional role.

In the Carter administration, for example, there were occasional disagreements between the State Department and the White House and Pentagon over how to deal with Moscow. The installation of Haig means the new administration will have a team of hard-liners throughout the key policy centers, and also that the potentially most influential voice for a stiffened foreign and defense policy may come from State, which in the past has been a spokesman for moderation.

Though Haig goes into the confirmation process as a source of controversy, it will not be the first time he has entered a key job in that situation and, if experience is any guide, it would be wrong to underestimate Haig's ability to come through it.

When he arrived at NATO headquarters in Belgium on Dec. 15, 1974, after Ford appointed him as supreme allied commander in Europe, there were many who feared that Haig -- though bright, attractive and obviously well-connected politically -- was too much the political-general and not enough the battlefield commander.

During his 4 1/2-year stay in that job, however, Haig won considerable respect among Europe's military and political leadership. He came to be viewed as a strong military leader who had made some major improvements in NATO's forces and was able to describe the Soviet threat with more intellectual force than simply yelling "the Russians are coming." He also proved a reasonably skillful diplomat who counts among his strengths the close relationship he developed with West German Chancellor Helmut Schmidt.

Haig's strategy in winning approval in Europe was basically one of keeping a low profile and saying relatively little in public. He has adopted the same strategy in gaining the nomination for secretary of state. Indeed, some senior Republicans say that Haig has said virtually nothing, even during the two major gatherings here of Reagan's interim foreign policy advisory group.

Toward, the end of his NATO stint, however, Haig started to speak out more in public. His main topic had become the Soviet threat and his tone had become increasingly strident. In some cases, it seemed to an observer, his remarks bordered an insubordination to President Carter, who had kept him on in the NATO job, and mocking in tone toward the president's Soviet policy. Haig, for example, repeatedly told audiences he did not see any evidence of Soviet military restraint at a time when Carter was using that yardstick to defend his decision not to deploy the neutron bomb.

Haig became this counry's most well-known general of recent years without ever having commanded a field army. But he did see action in Korea, where he took part in the Inchon landing, and commanded an infantry battalion and then a brigade in Vietnam in 1967, when he won a Distinguished Service Cross during the battle of Ap Gu.

However, it is Haig's loyalty to his bosses, his ability to work long, hard hours, to organize, to see that orders are carried out and to grasp issues quickly that have won him his political and military stars.It was a longtime Democrat, former Johnson and Carter administration official Joseph A. Califano Jr., who first spotted Haig in 1963 as one of a new breed of "renaissance men" and called him to the attention of then Defense Secretary NcNamara.

It was during his four years in the White House as deputy to national security adviser Kissinger, and then another 17 months as Nixon's chief of staff, that Haig flourished, in more ways then one.

It was there that Haig got involved in White House wire-tapping and in defending a president against an investigation that was closing in around the Oval Office. But it was there, too, that Haig earned widespread public acclaim for his role in helping get Nixon to resign, an effort that the late former counsel to Nixon, Fred Buzhardt, described as "one of the most skillful, tactful, diplomatic, brilliant and most sensitive feats in American history."

It was also in the White House that Haig undertook dozens of secret diplomatic missions to Vietnam, China, France and Cambodia that helped broaden him beyond the military role.

When he came back to the United States last year from NATO, Haig briefly thought of making a run for the presidency. He is an ambitious person and people close to him say he wants to be president. Having seen three presidents close-up, Haig may feel suited for the job. "He is an enormously competent man," says one top Reagan aide. "Yet he produces a feeling of uneasiness, something that's hard to put your finger on."