Alexander M. Haig Jr., the general turned diplomat who resigned yesterday in the most open split with his commander-in-chief since Harry S Truman's ouster of Gen. Douglas MacArthur, made no secret of his envy of MacArthur's ability "to operate with a degree of independence no longer acceptable."

"He was one of a dying breed of American bureaucrats . . . who could operate largely on their own," Haig once said in an interview. "It's a worrisome phenomenon to me. It's more difficult for our system now to produce this kind of guy, or for him to survive."

He did well to worry, because Haig was, by temperament, clearly a MacArthur kind of guy.

Only hours after taking office as secretary of state, the hard-charging Haig was offering President Reagan a memo to sign formally giving Haig primacy in all areas of foreign policy. When the White House staff--in the start of one of the capital's longest-running bureaucratic feuds--frustrated his initial power grab, Haig annointed himself the "vicar" of foreign policy anyway.

His quest for authority led two months later to a remark that became as widely quoted as any ever uttered by MacArthur, and more widely ridiculed.

After John W. Hinckley Jr.'s attempt on Reagan's life, Haig hurried to the White House Situation Room and held a press briefing. Asked who was making the decisions in government, he first misstated the Constitution's line of succession and then declared: "As of now, I am in control here."

But for all of the White House staff's efforts to cut him down to size, there never was any doubt during Haig's 17 months as secretary of state as to who was the major force in shaping the nation's foreign policy.

He was--as Haig himself described it--the primary "formulator, executor and articulator" of the Reagan administration's relations with the world: a highly visible presence denouncing Soviet militarism, warning of the communist threat in Central America and shuttling between London and Buenos Aires in an unsuccessful bid to prevent war over the Falkland Islands.

Ironically, until he took office as secretary of state and became obsessed with his "mandate" to move to the front of the foreign policy stage, Haig had been, in a way, the consummate insider.

An obscure colonel when he was selected to join the White House staff in 1969 in the rather routine post of senior military adviser to Henry A. Kissinger, the workaholic Haig quickly made himself useful as the man who provided the organizational backup for the peripatetic national security adviser. With Kissinger frequently on the move, Haig soon was working increasingly closely with then-President Nixon.

Both Kissinger and Nixon came to view Haig as close to indispensable. With their sponsorship, he quickly was promoted to brigadier general and then major general while serving as Kissinger's deputy. He was named Army vice chief of staff, skipping one full rank, in January, 1973. Many of the 240 generals passed over for the latter position voiced open dismay at his meteoric rise, noting Haig had become one of a handful of four-star generals never to command a division.

But if he lacked military command credentials, Haig clearly had impressed his commander-in-chief with his moves on the Washington battlefield. Only four months after he had been named to his new position, Nixon prevailed on Haig to return to the White House to fill the void left by the Watergate-inspired departure of H.R. Haldeman.

During the next year, as the stain of Watergate spread, Haig was widely credited with holding the White House together while Nixon fought to avoid impeachment. He displayed during this time some of the military imperiousness that dogged him a decade later as secretary of state.

As White House chief of staff at the time of the "Saturday Night Massacre" of October 1973, it was Haig who instructed acting attorney general William Ruckelshaus to fire Watergate special prosecutor Archibald Cox, telling him: "Your commander-in-chief is giving you an order."

Haig never seemed to put behind him this penchant for military idiom, causing a minor flap earlier this month when he likened his relationship to U.N. Ambassador Jeane J. Kirkpatrick--a fellow Cabinet member with whom he feuded regularly--to that of a general dealing with a "company commander."

While some have faulted Haig for his role during the Nixon years, he came through the period relatively unscathed. In 1974, President Ford returned him to military service, naming him commander of NATO. Haig met considerable skepticism initially from America's European allies, who felt they were getting a discard.

But he soon won their respect, both as a strong military leader and as an American who shared the European establishment's concern over the Soviet threat. It was during the Carter years that the public Haig began to emerge, attacking the Soviet buildup in a strident manner that some Carter administration officials viewed as close to insubordination.

Increasingly unhappy with the Carter administration's policies, Haig returned home in 1979 and explored the possibility of running for president. He soon abandoned the idea, however, and accepted a lucrative offer to become president of United Technologies.

Even though he underwent triple bypass coronary surgery in 1980, he insisted his health was no bar to accepting Reagan's offer to become secretary of state. As proof of that, the hyperactive Haig "relaxes" by playing a brand of tennis described as at least as aggressive as his diplomatic style.

While no one seems sure how Haig--the second of three children of a Philadelphia lawyer--decided on a military career, relatives said he wanted to be a soldier from the time he was four. A lackluster cadet at West Point, where he graduated 214th in a class of 310 in 1947, he began his active duty career in Japan where he was posted to the staff of MacArthur.

"I learned a lot out there," Haig said years later. "He was a very accomplished American leader at the end of a generation of leaders who could operate largely on their own."

That's the way Haig seemed to like to operate as well.