IT IS, SELF-EVIDENTLY, a matter of genuinely historic consequence for American foreign policy when the absolute ruler of a nation of critical strategic and economic importance to the United States is rather swiftly rendered powerless. And that is precisely what has happened in Iran over the course of a few, short, turbulent months of bloodshed, economic disruption and political protest. What this means is that a quarter century or so of Iranian stability and constancy -- for better or worse, as the shah's supporters or critics may choose to see it -- has ended. What it also means is that for Iran, and for that part of the world, the United States is going to be forced to develop and try to conduct quite a different and, almost certainly, more difficult foreign policy.

True, at this writing, it is not quite all over for the shah. But he has plainly failed in his attempts to assauge his disparate and determined opposition with tentative reforms, to silence it by jailing the dissidents -- or to crush it with harsh military repression. Now he is reduced to a last, desperate effort to hold onto some tiny pinch of power and at least some slim remnant of dynasty by negotiation with his opponents. His country is reduced to chaos. Power is in the street, or with the military.

In other words, we do not know who is winning -- or ultimately will win -- the Iranian revolution. We only know who has lost

But to know even that much is to know quite a lot. It is to know that in a region of almost incalculable political and economic importance to American national security, where once there was a familiar, reliable and influential ally, tightly tied to the United States by treaties, defense commitments and commercial arrangements, there will now be... who knows what?An unstable civilian regime, giving full expression to the violent anti-American sentiment so much in evidence in the movement against the shah, inclined to socialist experimentation, responsive to Soviet interests, perhaps heavily under Communist influence? A harsh military dictatorship, ruthlessly repressive of the more progressive forces that contributed to the shah's downfall? Perhaps nothing so extreme will evolve. What you can be sure of, though, is that Iran's future rulers will bear little resemblance to the government on which the United States has relied so heavily -- and lavished so much in arms and faithful attention -- through a succession of Republican and Democratic administrations stretching back to the early post-war Truman days.

What are Americans to make of this? The case can be made, with hindsight, that the United States staked far too much on the shah, most egregiously in the Nixon administration's open-ended, blank-check arms offers in 1972. President Carter will be faulted, no doubt, for not sensing the problem soon enough. Some will say he did not do enough to support the shah. Others will say that, in effect, he did too much, by (a) not trying to associate the United States with the more moderate elements among the shah's oppositin, or (b) not pressuring the shah to be more accommodating. He can be made to look foolish, as well, in his predictions, as recently as a few weeks ago, that "I fully expect the shah to maintain power in Iran..."

But we think no useful purpose will be served by a domestic debate over Who Lost Iran? We have the experience of the Kennedy administration with the Diem government in South Vietnam in the early 1960s -- an effort to influence internal events that began with the withholding of American aid and ended with the United States implicated in the coup that overthrew Diem, lead to his murder, and seriously disrupted the American war effort. In these matters, the alternative to looking blindly loyal, or merely foolish, is sometimes to be, or seem to be, downright treacherous.

In any case, there were good reasons, economic and geopolitical, for an American policy over the years that put an extraordinarily high value on Iranian oil and influence. That policy has been not so much discredited, in our view, as overtaken by internal Iranian events that were very largely beyond effective American control. The shockwaves from the collapse of the dynasty of shah Mohammad Reza Pahlavi could deal heavy blows to other politically and economically crucial monarchies in the immediate vicinity, just beginning with Saudi Arabia; to Israel, for whom Iran has provided not only oil but also political support; and very likely in one way or another, to the Western world.

So the urgent question now is not Who Lost Iran -- the short anser is that the shah did. The important question is what the United States now can do for -- or with -- its friends and allies to soften the blow.