In late October 1973, almost 11 years to the day after the Cuban missile crisis ended, U.S. intelligence reported that a Soviet transport ship passed through the Bosphorus from the Black Sea to the Mediterranean, headed toward Egypt, which then was at war with Israel.

The ship carried a radioactive cargo.

Central Intelligence Agency analysts suggested that the cargo could be warheads for Soviet short-range missiles that were then being delivered to Egypt.

The report, according to one official involved at the time, sent a "tingle" through the U.S. national security establishment at a moment when the United States was working to prevent a threatened unilateral Soviet military intervention in the then-raging Yom Kippur war.

James R. Schlesinger, secretary of defense at the time, said in a recent interview that since the Cuban missile crisis, the Soviet Union had "been very careful about moving any nuclear weapons outside" its territory.

So the 1973 movement itself was "a considerable change in the pattern of Soviet behavior," Schlesinger said, "and therefore tended to tell us that the Soviets were indeed politically serious . . . ."

But, he went on, "as to the possibility of the use of those weapons, we placed a very low probability on that."

Their purpose, "to the extent that one thought that the Soviets had a real purpose in making those moves, . . . was to bolster Israel's opponents," Schlesinger said.

Schlesinger, who previously headed the CIA, suggested an additional dimension to the Soviet move: "It was widely believed in the Arab world that Israel itself had nuclear weapons. These Soviet weapons were being moved into the region, it was speculated, in order to reassure either the Egyptians or the Syrians . . . that if the Israelis were to use such weapons, that there would be a response."

"It also was intended," Schlesinger recalled, "to deter any such Israeli action, if indeed the Israelis had it a nuclear weapon ."

Thus the stage was set for the last major nuclear confrontation between Washington and Moscow.

It illustrated a new danger: that the superpowers, who had learned from the Cuban missile crisis to avoid direct clashes between their own forces, could still be dragged into conflict while assisting allies.

This more complex form of Soviet nuclear diplomacy -- sending signals not only to the United States but also to Israel and its own Egyptian and Syrian allies -- turned out like earlier episodes. It quickly provoked an American response in the form of a worldwide strategic nuclear alert.

That alert was meant to be kept a secret -- except from the Soviets. But word of it quickly leaked and reached the entire world. Another U.S. signal at the time has never been publicly mentioned: a private message to the Soviets that said, according to a Pentagon official at the time, that the United States "might not be able to restrain someone else [in other words, Israel] from 'taking out' Soviet nuclear warheads." Nuclear 'Wild Card' Alters Equation

That 1973 incident illustrates why nuclear weapons have begun to lose their value for the superpowers as a tool of diplomacy. And the introduction of a nuclear "wild card" -- Israel -- may explain why there have been no more nuclear alerts since.

On October 6, 1973, Egyptian and Syrian forces launched a surprise attack against Israel, trying to regain lost Arab lands. As the tide of battle went back and forth, Moscow and Washington each tried to arrange a cease-fire to give its own client state the best result.

Early in October, shortly after the Egyptians had invaded the Sinai, the Soviets alerted several airborne divisions. Later, when U.S. aid was flown to Israel, the Soviets began flying ammunition and other heavy military materiel into Cairo.

By mid-October, Israel gained the advantage, and by Oct. 21, the Egyptian Third Army Corps, some 25,000 men and the cream of Egyptian President Anwar Sadat's army, was facing annihilation.

Soviet leader Leonid Brezhnev, seeing the possibility that Israel was about to score a major victory that could unseat Sadat, suggested that U.S. and Soviet forces move jointly into the area to enforce an earlier, agreed-upon cease-fire. Sadat gave his support to Brezhnev's proposal.

At the time, Brezhnev was maneuvering to restore Soviet influence in the Arab world, which had diminished badly less than a year earlier when Sadat had expelled thousands of Soviet advisers from Egypt.

The Brezhnev note came at a time of domestic turmoil in Washington, where the Watergate crisis was intensifying with new calls for the impeachment of President Richard M. Nixon. Days before, Nixon had fired Watergate special prosecutor Archibald Cox and failed to prevent Attorney General Elliot L. Richardson from quitting during what was called the "Saturday Night Massacre."

Richardson said recently that Nixon told him several times during their last meeting that his departure from the White House could harm national security.

"We felt that the Soviets had been watching what had been going on," Schlesinger recalled. "The Soviets, in view of their concern about the Middle Eastern problem, might conclude that the United States was incapable of reacting because of the domestic crisis. And that if they acted boldly, in view of our presumed preoccupations, that they might be able to score a major geopolitical coup."

The report that the Soviets were moving nuclear materials to the war zone arrived in Washington at the same time U.S. intelligence lost track of a number of Soviet air transports. The planes had been flying equipment into Egypt. Schlesinger said their sudden disappearance was interpreted as a sign that the Soviets had successfully put in place the necessary equipment for a Soviet intervention into Egypt.

Senior officials of the Nixon administration had received these disquieting intelligence reports by Oct. 24, when, late that evening, the Brezhnev message arrived declaring Moscow's intention to act unilaterally if necessary.

Senior officials gathered for an emergency meeting late that night in the White House Situation Room. Secretary of State Henry A. Kissinger chaired the session. Nixon did not participate. He said recently he had given Kissinger "authority to do what was necessary diplomatically to prevent Brezhnev from intervening militarily." [At the time, questions were raised about whether Nixon had ordered the alert as part of his scheming to save himself from Watergate, but the interviews for this article suggest that the alert was a response to what was perceived as a real threat, one carefully considered by senior offficials.]

"It was clear we had to react vigorously and there was no real debate about that," Schlesinger said. Kissinger suggested a full-scale, worldwide, nuclear alert "so as to make the greatest impression on the Soviets."

"It was our belief," Schlesinger said, "that Mr. Brezhnev would be prepared to back down because he was looking for an easy opportunity to move into the Middle East."

"The United States," he added, "and indeed all of mankind have been in the fortunate position that Soviet leaders over the course of some 30 or 40 years have been prudent and cautious men . . . . Leninist doctrine tells them not to indulge in adventurist schemes -- to wait, because time is on their side."

But, he added, "That does not mean that the Soviets are so prudent that they refrain from pushing . . . . We must be assured that our forces will survive and have the capability to take the appropriate actions."

At that delicate moment in 1973, the United States responded to Soviet moves by going on alert and privately warning the Soviets that the Israelis, too, were on alert. Washington also "brought great pressure on the Israelis to adhere to the spirit and the letter of the cease-fire," Schlesinger said.

The next day, Brezhnev agreed to a United Nations force without superpower representation to insure the cease-fire, dropping his threat to intervene with Soviet forces. The transport with nuclear cargo, which had been tracked on a course toward the Egyptian port of Alexandria, turned back and went elsewhere, a former top CIA official said this week.

Arkady Shevchenko, the highest-ranking Soviet diplomat to defect to the West, said that to Soviet leaders, faced with the choice of risking nuclear war or saving Egypt, "the choice definitely was forget . . . Egypt and avoid the risk of a nuclear war."

The U.S. alert had "actually a sobering effect on the Soviets and the next day they were already not talking about any kind of joint operation or even the possibility of use of Soviet conventional forces," Shevchenko said.

Reflecting on this episode, former president Nixon said in a recent interview that "in order to avoid it escalating to the nuclear level, we felt that it was important to make it very clear to Brezhnev then that if he moved in, we would react." But, he added, "we did not at that time threaten to use nuclear weapons. That was in the background."

Speaking more broadly about the superpower standoff since Hiroshima, Nixon held up nuclear weapons and nuclear diplomacy as "the major factor s today, as we look back over 40 years, that have prevented World War III."

Former defense secretary Harold Brown said recently that he "would not ignore the effect of nuclear terror as a deterrent to conventional war. I think it has been real . . . . I think it has contributed to the caution which both the U.S. and the Soviet Union have shown when it's come down to the possibility of engaging each other's military forces."

"I don't think there is any magic solution to this," Brown said, "nuclear weapons cannot be un-invented . . . and that means we have to go ahead trying to manage deterrence by the threat of retaliation . . . and manage the political relationships so as to ty to minimize the chances of a confrontation . . . ." Arms Production Continues Unabated

While the military and even diplomatic value of nuclear weapons appears to have declined since Hiroshima, the production of weapons by each nation has continued unabated.

The atmospheric test ban of 1963, negotiated and signed in the post-Cuban missile crisis era, may have promoted rather than inhibited the arms race. With weapons tests driven underground, the public no longer was told the size of the blasts, no longer had brilliant pictures to contemplate and was not regularly shaken by reports of rising radiation levels in the atmosphere.

Since the early 1960s, nuclear bomb and missile warhead stockpiles on both sides have soared -- from about 6,000 U.S. and 500 Soviet strategic weapons (including bombs) then, to roughly 10,800 U.S. and 9,600 Soviet warheads now.

Though the history of the nuclear era has demonstrated repeatedly that these weapons were not considered usable in crises, the numbers have taken on an importance of their own, often influencing not only diplomatic discussion but also domestic politics in both Moscow and Washington as symbols of the relative ascendancy of one superpower or the other.

U.S. and Soviet leaders have been "notably prudent and careful" dealing with the risk of conventional and nuclear war while in office, former national security affairs adviser McGeorge Bundy said. But "I don't think we can be as comfortable about the kinds of choices that they have made, or at least approved, in the procurement and deployment of weapon systems which create reciprocal fears and continuing competition," Bundy added.

Robert S. McNamara, secretary of defense in the Kennedy and Johnson administrations, recently criticized both superpowers for continuing to produce nuclear weapons on top of some 50,000 strategic and tactical weapons that now exist.

"I think I can say without any contradiction," McNamara said in a recent interview, "that there is no piece of paper in the world that shows how either the Soviets or the U.S., Warsaw Pact or NATO, can initiate the use of these warheads with advantage to itself . . . . They have no military purpose whatsoever excepting only to deter one's opponents from their use, and we surely don't need 50,000 for that purpose."

The Soviets, however, see another purpose for nuclear weapons.

Shevchenko summed up what is a prevailing U.S. government view that "the Soviet leadership considers that the stronger they are, militarily and especially strategically," the more easily they can achieve political goals.

"Without this enormous military, strategic, nuclear arsenal," he said recently, "the Soviet Union would not be a superpower. It would be behind Japan. It would be behind even Germany."

Nixon sees the present growth in Soviet nuclear weaponry as posing a potential new danger to the equilibrium between the superpowers that has helped keep the peace for four decades.

"Superiority in nuclear weapons for a defensive power keeps the peace," he said. "The United States had it for almost 40 years. And we never used it for offense. We always used it to deter war." U.S. nuclear diplomacy, he said, deterred Soviet diplomatic probes in the 1950s, the 1960s, and in 1973.

"Superiority for an offensive power like the Soviet Union," he went on, "a power that is trying to extend its domination, risks the peace."

Not that Nixon foresees the Soviets using their weapons. "The men in the Kremlin are not madmen. And they're not fools. They do not want a Europe that has been atomized. They don't want a United States that has been destroyed . . . . The great danger insofar as the West is concerned is not destruction in nuclear war, but surrender through nuclear coercion," Nixon said.

Others disagree that nuclear coercion is a serious prospect. Bundy, for example, said in an interview that nuclear diplomacy has "really never worked for either side at any time.

"Nuclear danger," he said, "has created the natural caution at a number of points for many countries, including our own. But the notion that you could use a nuclear monopoly, or even less, a marginal nuclear advantage for the purpose of getting things your own way in some disputed area is not, I think, borne out by the historical record. Quite the opposite."

Like others, Bundy doubts that U.S. nuclear superiority at the time of the Cuban missile crisis was significant to the outcome of that episode. U.S. conventional military superiority in the Caribbean plus a workable diplomatic option ended the Cuban crisis, Bundy believes.

Nixon said he believes that today's greatest danger is "what happens when these nuclear weapons come into the hands of smaller powers." That view is widely held among former U.S. officials.

Schlesinger said, "I fear that if we do ever see a nuclear weapon used in anger, it is likely to be in the Third World."

Nixon said that deterring the spread of nuclear weapons is a common Soviet and American interest.

"Both the United States and the Soviet Union," he said recently, "don't want these nuclear weapons to be proliferated all over the world because some nut like [Libyan leader Muammar] Qaddafi, you know, may set off the whole world . . . . "