Forty years ago today, Harry Truman made a speech and the United States became a superpower. Of course, at the end of World War II the United States was a superpower, and with a nuclear monopoly, the superpower. But with the enunciation of the Truman Doctrine on March 12, 1947, the United States finally accepted the role.

There could be no more pretending that the peace would be kept by others. In fact, the Truman Doctrine was precipitated by two British notes sent on Feb. 21, 1947, to Secretary of State George Marshall. Britain, Europe's perennial balancer-of-power, informed the United States that it could balance no more. It would stop aid to Greece and Turkey on April 1.

Greece and Turkey were near collapse. Greece, fighting a civil war against communist guerrillas, was near bankruptcy. Turkey, subject to repeated demands by Stalin for bases and for rights to the Dardanelles, would be the next domino to fall. (The sound of dominoes emanating from Eastern Europe was loud in 1947.) An exhausted Britain could no longer carry the burden. Truman had five weeks to act.

Within 19 days, he had remade American foreign policy. He asked Congress for $400 million in economic and military aid for Greece and Turkey, an astonishing and unprecedented step for peacetime America. But Truman did not stop there. He declared it the policy of the United States "to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures." This universalization of America's role was immediately called the Truman Doctrine. With it, "containment" was born. The Monroe Doctrine had pledged the United States to keep foreign powers out of the Western Hemisphere. The Truman Doctrine pledged the United States to contain Soviet power and preserve freedom wherever it could around the world.

When the speech ended, said a witness, Congress' "applause had a bewildered quality about it." Initial reviews were not all favorable. Conservatives, such as Sen. Robert A. Taft, argued that Truman's ideological commitment to defend "free peoples" would lead either to world war or to bankruptcy. Liberals, such as Claude Pepper in the Senate and Henry Wallace in The New Republic, objected first that "our" Greeks and Turks were reactionaries unworthy of support. They objected, second, that Truman should have turned the problem over to the United Nations, yesterday's Contadora.

In the end, however, Truman won. By May 15, the aid was approved by both houses of Congress. The Truman Doctrine saved Greece and Turkey. And coupled on June 5 with the Marshall Plan, it saved the rest of Western Europe.

The Truman Doctrine was the guiding principle of U.S. foreign policy for a generation, until the consensus for containment disintegrated with Vietnam. Several stopgaps were immediately offered as replacements. First was the Nixon Doctrine: relying on friendly regimes to police their regions on our behalf. The great model was the shah. The Nixon doctrine fell with him.

Then, after the invasion of Afghanistan, the Carter Doctrine promised unilateral American action to defend Western interests in the Persian Gulf. This doctrine died quickly for lack of credibility. The Rapid Deployment Force, sword of the Carter Doctrine, is hardly able to repel Iranian mullahs, let alone the Soviet Army.

If regional powers prove unstable, and projected American power unreliable, what then? As Joshua Muravchik points out in the winter issue of Foreign Affairs, global containment -- the idea of resisting the Soviets everywhere -- collapses and gives way to a new American policy vis-a`-vis the Soviet Union: selective containment. Some countries the United States will support against Soviet-backed forces. Others not. In the late '70s, for example, we said no to Angola and yes to El Salvador.

There is one problem with selective containment. Alone, it is a policy of continual retreat. If the Soviets gain a foothold in, say, Angola, they keep it. The Brezhnev Doctrine enforced by Moscow pledges that Soviet advances will not be reversed. Selective containment plus the Brezhnev Doctrine means: what's theirs is theirs and what's ours is up for grabs. The fight is always on Western terrain.

The strategic response to this asymmetry has come to be called the Reagan Doctrine. It says that recent Soviet acquisitions at the periphery of empire -- Angola, Afghanistan, Nicaragua -- are not permanent. They are open to challenge. And we support the challenge. The Reagan Doctrine declares overt U.S. support for the anti-communist resistance movements. By declaring Soviet gains reversible, it saves selective containment from being a policy of gradual, but inexorable, retreat. It thus reestablishes an equilibrium -- a dynamic equilibrium -- in the strategic equation between the United States and the U.S.S.R. Moreover, by not requiring global resistance at every point, but allowing the United States to choose its terrain, it restores initiative to the American side.

American foreign policy is routinely criticized for its reactive quality, for its air of ad hoc pragmatism. Hence the demand for initiative, strategy, some larger vision of how to deal with the world and with the Soviet challenge. Containment and the Truman Doctrine met that demand exactly 40 years ago. Selective containment and the Reagan Doctrine meet it today.

The Reagan Doctrine may, nonetheless, be undone by the Iran affair, by the zealotry of those who acted secretly and perhaps illegally just as Congress was coming to open military support for the major anticommunist insurgencies. Congress, it seems, will have its revenge. If so, those preparing to defund the Reagan Doctrine might inform us of their alternative strategy for dealing with Soviet advances around the world. Or shall we be content with a policy of gradual retreat? Harry Truman didn't think so.