Can you catch a falling Shah?

The documents captured in the American Embassy in Iran, now being published in some parts of the United States press, again raise the question, ''If we knew the Shah was in trouble, why didn't we do something?''

From the documents it is clear that the Foreign Service reporting from the embassy was telling increasingly of growing opposition to the Pahlavi regime, not only among religious leaders but among bazaaris and middle-class groups as well. The handwriting seemed clear.

The hard question is not ''Why didn't we do something?'' The more difficult question is ''What could we have done?'' In most situations of this kind, there is no dearth of intelligence reaching the president. There is only a dearth of solutions.

There is probably no more agonizing foreign policy problem facing a president than dealing with a friendly regime in trouble. The issue is even more aggravating when the US is, as in the case of the Shah, closely identified with both the person and the regime.

In Iran the stakes were high, strategically, politically, and economically. The Gulf states were watching Iran; they were watching the response of the US. Oil was always a factor. No president could be unmindful of the domestic US political consequences of the collapse of a major regional ally.

The first problem a president faces is to evaluate the information. The Foreign Service and CIA reporting may be only part of the information he is receiving. In the case of Iran, there were conflicting reports on the cohesion and readiness of the military, a key element in the Iranian political picture. Adherents of the Shah were discounting reports of opposition to him. Within the US government, components of the intelligence community could not agree on the conclusions of a national intelligence estimate during the critical months before the Shah's collapse. Presidents and their advisers, further, are often suspicious of Foreign Service reporting which they feel may be too sympathetic with opposition elements.

Even if a president should conclude that a friendly regime is in deep trouble , what are the options open to him?

He can talk with the ruler about his internal situation. The chances are that the ruler will be less than candid, will seek to portray reports of unrest as either exaggerated or as evidence of a communist plot. There is the risk that the US government will be asked for a level and type of support it cannot provide.

He can make suggestions to the ruler in trouble. To do so could lead to greater involvement. Suggestions for further democratization or reforms either may be too late or may add to the pressures against the regime. Suggestions of harsh repressive actions could risk strong oppostion in the US and further identify the US with the worst features of a failing regime.

Talks can be opened with close foreign allies about the situation in the country in question. Efforts might be made to mediate with moderate opposition groups. Concerted action with allies is possible, however, only if the respective estimates of the situation are close. The allies are frequently less enthusiastic about active involvement than we. Perhaps the minimum possible is to discuss contingencies in the event of a total collapse.

Military measures by the US are one possible option. The movement of a naval force or the sending of an air unit on a visit might serve to show support for the ruler at a critical time and to deter actions against him. Such moves, at the same time, run the risk of exacerbating a situation which may already be getting out of control.

In some situations, foreign rulers may feel that a change of attitude on the part of the US toward a regional problem can help that ruler's chances. Arab leaders in difficulty have often pled for a change in US policy toward the Arab-Israeli problem. US presidents have generally expressed doubt that changes in American policy are likely to help a foreign ruler already in trouble -- even if such changes in policy were possible.

A president, conceivably, could take another tack. He could conclude, on the basis of intelligence reaching him, that the regime could not survive or, at least, that its chances were slim. He could begin preparing for the future by opening talks, either through overt or covert means, with future leadership. Such talks would almost certainly be known to the ruler. If they were not a part of our earlier pattern, they would send a signal of lessened confidence. The action would be likely to cloud our dialogue with the ruler at a critical time.

Any presidential action at such a time faces the almost certain likelihood that it will become public. Public knowledge of a change in the attitude or policy of the US may accelerate forces and bring on the very results this country is seeking to prevent.

The cases are hard to recite in which the US has successfully saved a foreign ruler beset by internal weaknesses and rising disorders. There are many cases where regimes with which the US was closely identified could not be saved. The reason lay much less in the absence of pertinent intelligence than it did in the absence of viable ways to deal with the situation described by the intelligence.