Three months have elapsed since the Soviet invasion of Afghanistan. So there's reason for an inventory of the actions taken in the wake of what the president called "the greatest threat to peace since World War II."

Against the Russians, the United States has declared an embargo on grain sales. But Argentina and Canada has not gone along, and some third countries are buying in the United States and transshipping to Russia and its clients. So the harm to Moscow is small.

Severe limits on sales of high technology to the Soviet Union have also been put in place by the United States. But other advanced countries are doing businesses as usual with the Russians.

At the U.N. General Assembly, and in the Islamic Conference, lopsided majorieties denounced the Soviet action in vigorous terms. A third or more of the 140 countries due to participate in the Moscow Olympics will take their marbles elsewhere.

Still, these are largely condemnations of a moral kind. They are not going to winkle the Russians out of Afghanistan or even soften the takeover. The less so as Britain and France (with the usual backing from the State Department) have been trying to pull the Russians into a dialogue about a "neutral" Afghanistan. The Russians, with dirty work still to do in that land, have turned a deaf ear. But Moscow is on notice that it can renew the dialogue with the West anytime it wants -- and on practically any terms.

Pakistan is the country next in line after Afghanistan. The United States rushed in with offers of military and economic assistance. On examination these offers turned out to be the kind Pakistan could not afford to accept. The Pakistanis have decided they could do better by cutting a deal with Moscow and New Delhi.

Saudi Arabia, and the southwest coast of the Persian Gulf, now emerge as the true stake, the great prize at issue in the present crisis. But even as its crucial importance becomes clear, so does the vulnerability of a weak, archaic and corrupt Saudi regime, exposed to the pressures of both militant Islam and radical forces.

Efforts to shore up Saudi Arabia have been launched. A move to push Palestinian autonomy in a way that will ease the subversive pressure on the Saudis is under way in the Egyptian-Israeli talks. The United States is acquiring naval and air facilities in the northwest quadrant of the Indian Ocean. The fleet there has been reinforced, and a battalion of Marines sent aboard.

But these also are Band-Aids. Even those who seek any stick to beat the Jews cannot seriously believe that a grant of autonomy to the Palestinians would save the Saudi regime. Or American bases in distant countries, or the fleet and Marines in the Persian Gulf.

The threat to the Saudis comes from the radical forces in the area -- the left wing of the Palestine Liberation Organization, the communists in South Yemen, Libya and its subsidization of terror and subversion and the Cuban expeditionary force.

A coherent policy for inhibiting these troublemakers, however, has not been developed by the United States. Nor has this country called on its allies for help in this area. On the contrary, the French especially, and to a lesser extent the British, Germans and Japanese, are busy patronizing the radicals and their paymasters in the hope of achieving better terms for buying oil or selling arms and capital goods. So the very heart of the problem -- the provision of security for the southwest coast of the Persian Gulf -- remains in doubt.

What emerges from all this is a policy on the edge of collapse. The Russians have asserted themselves as the predominant power in the wortex of world politics. Their radical clients are on the march. Afghanistan has passed under Soviet control, and two main countries in the area -- Iran and Pakistan -- have taken themselves outside the fold of American protection. The capacity of the United States to secure Saudi Arabia reveals itself as doubtful at best.

Perhaps the tide can be turned. Maybe the United States will conceive -- as it conceived in the Berlin airlift in 1948 -- of a sure way to defend the Saudis. But as of now, 90 days after its enunciation, the principal content of the so-called Carter Doctrine consists of scouring the world in pursuit of alibis for doing nothing.