The most surprising thing about the demise of Alexander Haig as secretary of state is that it was so long in coming.

He was a high risk choice for the job, as a New York Times editorialist wrote when he was named by President-elect Reagan. Within a month of taking office, his fatal weaknesses were in sight, along with the startling strength of his assertiveness and drive. In another month, by late March of 198l, it was clear that he did not and probably could not fit into the collegial top rank of Ronald Reagan's administration.

Early on, the press corps at State sensed the complexity of the man -- and of his situation. It quickly became a wary relationship, with notably less rapport, warmth or mutual confidence than had been the case with other secretaries of state in recent years.

At the very first, he came on stronger than anyone I ever saw in Washington who had not been elected president. He was so sure of himself and his dominance of foreign affairs, more like president for foreign affairs than a mere mortal who had been hired (after he campaigned hard for the job with Reagan's "kitchen cabinet") and who could be fired like anybody else. He not only assumed the role of "vicar," as he proclaimed, but clearly he thought of himself as the pope.

He opened his first press conference, eight days after taking office, by quoting the Bible that even the universe took seven days of creation. He spoke proudly of "my

nominees" -- he repeated the words for emphasis -- "my nominees" for high State Department offices, and went on to describe "our first official state visitors," a list of presidents, prime ministers and kings, without bothering to mention that Reagan had a role in making the nominations and being host to the state visits.

It couldn't last, and it didn't. On Feb. 20, 1981, a man who had been an insider in the Reagan campaign and transition told me that Haig, while impressive in his capacity to grasp and assimilate quickly, was using up his credit very fast with the inner circle. The problem was his over-assertiveness, and he quoted two close advisers in the telling.

After my discussion with the insider, I wrote in my notebook, "Among the crucial questions of this administration is the relationship between the real world and some of Reagan's strong political instincts and positions. That connective link is provided by Haig. . . . And a headstrong, ambitious Haig has liabilities as well assets in this role: the more he seems effective, the more he 'wins,' the more endangered will be his position at the White House."

The next note turned out to be more prophetic than I imagined: "The role of Judge Clark can be crucial. He is a real insider."

As long as Clark was at Haig's side as deputy secretary of state, he ran interference for "Al," as he called him. On the night of last March 24, when the White House openly rebuffed Haig by naming Vice President Bush to be "crisis coordinator" over Haig's public objection, the secretary of state wrote out his letter of resignation and took it to Clark. "The Judge" convinced "Al" not to take it to the president, on grounds that Ronald Reagan does not like to be crowded.

In the final episode 10 days ago, Haig again took his complaints to Clark, now installed in the White House as national security adviser, but the results were very different. Some say by then Clark's patience with Haig had been eroded. An equally plausible explanation is the old Washington bureaucratic adage, "Where you stand depends on where you sit," and now Clark has a presidential perspective.

The secretary of state, in the governmental landscape, lies between the foreign policy ideas, objectives and even pledges of a president, on the one hand, and the world beyond our shores full of people and nations with ideas and interests of their own. As the man in the middle, he spends much of his time and more of his credit in representing the world outside to the U.S. government than the other way around.

It is the nation's chief diplomat who is compelled to say, if he dares, that this or that dearly held idea cannot be accomplished abroad, or cannot be done in the way which is envisaged. Even with good personal relations, presidents and their staffs are forever grumbling about the intransigence of their secretaries of state and their legions of cookie-pushing aides.

The holding of this centrist ground, especially in a non- centrist administration, was Haig's central accomplishment. As a former general and a certified hard-liner on the Soviets, he was in position to turn aside or postpone proposals that would have brought crises in several areas: East-West and alliance policy, arms control, China-Taiwan policy, among others.

Curiously, in the arena of Central America, Haig was the most eager for a radical turn and, in a reversal of roles, had to be restrained by the Pentagon and the White House.

Because it was his job to implement policy, he understood that the United States relies on allies and alliances that cannot be ignored. President Reagan's turn toward unilateralism, especially in East-West economic issues, was part of Haig's unhappiness at the end.

When it came to positive rather than negative accomplishments, especially in the political area, there was less to show. For example, his idea of a Middle East "strategic consensus" against the Soviets was a bust from the start, and he was never able to develop a positive policy to continue the momentum in that crucial area. Only in the Namibia negotiations in southern Africa did Haig's State Department show the persistence and energy needed for political success, and that effort yet may founder over the U.S. injection of the Cuban troops issue.

As a military man, it issnot surprising that Haig often seemed most gripped by arms, arms policy and the potential and immediate clashes of arms. He surrounded himself with former military officers, and accelerated a shift in emphasis from economic assistance to military and security supporting assistance abroad. Hodding Carter, the former State Department spokesman, asked in a telling early comment: "Al Haig is a fine general, but why does Washington need two Pentagons?"

As for the press, Haig's well-known misuse of language reflected a lack of respect for words and those who live by them. He made inflated and even embarrassing claims for his efforts and often denied obvious truths when speaking to reporters. Some of his grave political problems came from careless words of his own: the public issue of "crisis management," the assassination day statement that he was "in control," his charges, through a spokesman of attack from a "guerrilla" in the White House.

He wanted to be liked by the press, as most public figures do, but seemed pathetically unable to find a way to do it. His often repeated statement, in the face of stories not to his liking, was, "I know you guys are writing what you're told," as if reporters were empty vessels to be filled at will with one or another faction's one-sided versions. He was often disappointed that "his" press corps, at the State Department, did not do battle on his behalf with "their" press corps at the White House, which had become a conveyor belt for officially inspired attacks on Haig.

Deeply affected by his experience as Henry Kissinger's deputy in the Nixon era, Haig made clear from the beginning his belief that foreign policy should be run from one place, either the State Department or the White House. If the former, he would dominate; if the latter, he would depart. But nothing is so clear-cut in today's Washington.

His horse began to bolt, and the terrain got rougher, but Haig tried to hold firmly to the reins. He had a longer ride than seemed possible at several turning points along the way. Given personality and policy differences, probably no amount of added effort could have kept him on top. Horse and rider probably were mismatched from the start.