A SIDE FROM keeping the nuclear peace, the most demanding task of American foreign policy since World War II has been to cope with Third World change. At various times and places, the United States has sought to stand firm, at others to go with the flow. But probably nowhere has it tried harder than it currently is in El Salvador to preempt revolution by reform. That is the special significance of the American effort to help bring progress and order out of the extraordinary convulsions there.

The tipping point in El Salvador came toward the end of 1979 when, with terror spreading on both the left and right a pretty promising centrist coalition or junta fell apart. Centrist Salvadorans found themselves forced to decide whether to look right and form a new coalition with military elements ranging from the responsible to the reckless, or whether to throw in on the left with revolutionaries and the worker, peasant and student organizations increasingly under their influence. Among the reformminded there was a grave awareness that, either way, they would have trouble taming the more violence-prone of their comrades.But they thought they could do it, or they yielded to what they felt was the rush of history. They chose and, in so doing, split the center.

American policy-makers have been laboring to rally Salvadorans of the center and center-left to the side of the government junta. That is the way to strengthen the civilians' weight in it and to enhance the prospects of effective reform. The going has been rough, but the United States has found it politically more Feasible and ideologically less objectionable to support reform, even reform soiled by some repression, than to condone revolution, especially revolution stained by nihilism. For its pains, the adminstration has suffered the scorn of the right, which is outraged by reformist thrust of American policy, and the pity of the left, which is anguished at the appearance of American complicity with the bad old oligarchy.

It is a difficult policy to conduct and explain, and it may fail: it takes a real optimist to believe that the center in El Salvador will hold. What those who spurn the junta seem to us to ignore, however, is that they are helping spin the country toward a civil war that will make the current carnage look like kid stuff. The junta the United States is supporting has far to go to contain its own violence, broaden the political process and deepen economic development. To think that those objectives could be effectively pursued by dumping the junta, however, is a dream.