Once more into the breach. Vice Adm. John Poindexter (you remember Poindexter) has replaced Robert McFarlane as national security adviser to the president. McFarlane replaced William Clark, who replaced Rich- ard Allen. That's four in five years -- or as many national security advisers since Reagan took office as there were under four different presidents in the preceding 20 years.

You could find no more telling a commentary on the Reagan administration's grasp of the problem of managing foreign policy and national security affairs. Ironically, the president all but made the point when he announced the admiral's appointment in the interest of "continuity." That is precisely what has been most lacking: a strong figure in place long enough to establish the right working relationships for one of the most powerful and precarious positions in government.

The personalities are not necessarily the issue. Poindexter is known as an able and highly intelligent insider with experience as a No. 2 man. The trouble is with the revolving door.

Allen left quickly under circumstances that were, well, awkward. Clark took not much more time to decide that he wanted a change. McFarlane cited personal considerations; friends said it had to do with combat fatigue after a year in a job that others had toughed out for more than four years.

We will now be treated to the usual "real story" -- the who-struck-John chitter-chatter that makes Washington a Fun City. There will be some truth to some of it, particularly that part having to do with the overbearing concern of White House Chief of Staff Donald Regan for territorial rights out of all proportion to his familiarity with McFarlane's territory.

Regan's pushing and shoving to front and center at the president's right hand in Geneva last month would have been enough to sour any national security adviser, the more so as McFarlane, profiting from his experience in the pressure cooker of Henry Kissinger's staff, had begun to demonstrate a genuine flair for the job. But that still isn't the point, which has to do with the breach between the various power centers of foreign policy-making, principally at State and Defense, that has made a common, coherent, consistent course of action uncommonly difficult.

The point, in short, has to do with Ronald Reagan.

For all his many gifts, he has yet to demonstrate even a rudimentary understanding of how things work or what has to be done to make them work. And so he has clung, by and large, to a concept of Cabinet government he developed as governor of California. One of its loudest advocates in the early days was none other than Secretary of Defense Caspar Weinberger, and you could hardly blame Reagan for believing that an arrangement that served him well in Sacramento was bound to work to his advantage in a laid-back presidency offering easy access to old and close associates.

But with all respect to the complexity of domestic issues and domestic politics, the business of conducting relations with foreign adversaries and sovereign nations requires a more complex management and a predominant role for the secretary of state. Alexander Haig staked out his claim to that "vicarship" in so flamboyant a fashion that he lost the preeminence he thought he had been guaranteed in advance.

George Shultz has played it in his dogged way with more success. But somewhere in the inevitable rough and tumble of any foreign policy-making process there has to be somebody in the White House who is answerable not to a particular bureaucracy but directly to the president -- somebody to sort things out and present competing "options" without personal prejudice or preference, at least until an opinion is requested. It is difficult and delicate work, the more so when a national security adviser is licensed by the president to take into account domestic political considerations as well. Finally, it requires a president who knows what he wants and how he wants it to work.

It worked for Henry Kissinger because from his early transition talks with Richard Nixon it was apparent that Nixon knew how he wanted to operate: from the White House. It worked for McGeorge Bundy and John F. Kennedy in a different way but for the same reason: they had an agreed sense of how to go about it. John Foster Dulles and Dwight D. Eisenhower did it their way. And so, with mixed results and some considerable friction with the bureaucracies, did Jimmy Carter and Zbigniew Brzezinski.

For better or worse, then, over the past half-dozen presidencies there has been a pattern of operation, of working relationships strong enough to endure in most cases for a full presidential term. How it will work for Poindexter is not entirely clear. But old hand Brzezinski was quick to spot a portent: Poindexter said he had been assured of access to the president -- assured by Donald Regan. An assurance relayed from Reagan to Re- gan, rather than directly from Reagan to Poindexter, sounds like an assurance of precisely the sort of malpractice that, despite all the denials, had begun to wear McFarlane down.

If that is so, Reagan has lost more than the services of an increasingly competent and confident national security adviser with the departure of Bud McFarlane. He has lost, as well, yet one more opportunity to fix something that plainly needs fixing.