Events in Tehran decree the opening of a full-scale inquest into American policy toward Iran. Not only because American citizens have to be protected. More important, the whole Mideast -- and in the matter of oil, the whole world -- needs to be insulated against the fanaticism of the Aytollah Khomenini.

The starting point of the inquest is acceptance of a hard fact that does not go down well with moralists in the country at large and the administration in particular -- namely, that the downfall of the shah was a calamity for American national interests.

Like him or not, the shah made available to the world a regular flow of oil in large amounts and without political conditions. His imperial pretensions, and the forces he maintained, imposed order on the Iranian plateau and the submerged nationalities that populate its borderlands.

The ayatollah, in contrast, instituted what amounts to a cultural revolution. He is preparing an Islamic constitution that vests absolute authority in the Moslem clergy, or mullahs. He pushes Iran's brand of Islam -- the Shi'ite version -- into neighboring countries where the more orthodox, Sunni brand of the creed holds sway. He ties the sale and production of oil to narrow political considerations.

Since theocracy doesn't work, partical power gravitates elsewhere. In Tehran, militant students and revolutionary guards rule the streets. The minorities of the provinces -- including the oil fileds in the south -- are restive or in revolt. Every country adjoining Iran sees in the ayatollah a threat to stability, which is not high in the region anyway.

Washington has tried appeasement of the ayatollah with disatrous results. Not only has American sovereignty been trampled in the embassy, but this country has been increasingly regarded by Iran's neighbors as a poor credit risk, a pitiful helpless giant.

In trying to right the balance, Washington first needs to reduce its vulnerability to the blackmail tactics of the Iranian militants. That means closing down the embassy in Tehran and shutting down the embassy here -- at least until a firm understanding on ground rules can be reached.

In the bargain, as an indication that it means business, the United States ought to enforce seriously the laws governing Iranian students in this country. If the laws are inadequate, then new statues can be written.

A next necessary step is to blunt the Iranian oil weapon. The United States can easily push the few American companies that do business with Iran to other sources. Whatever is lost on that account, and it will not be more than 500,000 barrels daily, can be offset from allied countried -- notably the Japanese, French and Germans -- that buy Iranian oil. In that way this country would show Iran and the world something they both need to know -- that Tehran cannot organize an oil embargo against the United States.

The next step is the truly important step. It involves finding occasion for an unmistakable, and preferably surprising, assertion of American power on behalf of the regimes that feel menaced by the ayatollah. That might take the form of supporting Iraq in its efforts to stir up provincial resistance inside Iran. It might mean giving military assistance to Turkey.

But the most likely candidates for American help are the oil-rich monarchies of the Persian Gulf. The regimes in Kuwait, Saudi Arabia, Bahrain, the Arab Emirates and Oman feel their security in danger. They would like to develop a joint policy for the Persian Gulf, and they met to consider measures to that effect in Saudi Arabia on Oct. 16.

The United States could support them en bloc. More dramatically and more effectively, it could give assistance to a particular country in a particular place -- say, Oman, at the choke point of the Gulf, the Straits of Hormuz.

To find and exploit such opportunities, there is required a crucial internal change in Washington. The united States needs a capacity to do something besides sending Marines and bombing. It has to rebuild a capacity selfdestructed only a few years ago -- a capacity for convert intervention.

As that requirement suggests, the rebuilding of American policy toward Iran is going to be a long, slow, uphill task. It will probably be years before this country is ready to play a political role in Iran.

But in the meantime there is one thing ordinary Americans can do. There is nothing in Constitution that obliges this country to be run by a president whose instinctive reaction to a challenge is a declaration of national impotence.