On Saturday, Oct. 27, 1962, one day before Nikita Khrushchev offered to withdraw Soviet nuclear missiles from Cuba, President John F. Kennedy approved plans for air strikes on Soviet nuclear missile sites, air bases and Cuban-Soviet antiaircraft installations on the island, all to take place the following Monday. Kennedy and his colleagues on the "Executive Committee" (Excom) of top officials convened to handle the crisis decided that an invasion of Cuba would follow, according to participants in the decision and notes from meetings in the White House. In other words, the Cuban missile crisis -- long regarded as the most dangerous moment in the nuclear age -- brought the two superpowers considerably closer to the brink of war than has previously been reported. For almost an entire day, the United States was actively preparing for military attacks on Soviet installations in Cuba.

At the time, U.S. intelligence believed that 20 of 24 medium-range Soviet nuclear-warhead missiles on the island were operational, with more becoming combat-ready each day.

The Joint Chiefs of Staff had told the Excom that the planned bombing raid could not be expected to destroy all the operational Soviet missiles on Cuba, so the United States faced the possibility that at least one missile could be launched, causing "almost certain . . . chaos in part of the East Coast" of the United States, as then-Defense Secretary Robert S. McNamara put it during one of the first White House meetings on the crisis.

Soviet air defenses on Cuba, which had shot down a U.S. U2 photo reconnaissance plane on Oct. 27, 1962, were growing stronger daily, making it increasingly more difficult to carry out a strike against the Soviet nuclear force, or even keep track of it.

"The actions that we took on Saturday," McNamara said in a recent interview, "were actions that could have led, might have led to a Soviet military response.

"I recall leaving the White House that night," he added, "walking through the gardens of the White House to my car to drive back to the Pentagon and wondering if I'd ever see another Saturday night."

According to the recently released notes of that White House meeting on Oct. 27, McNamara told the Excom that "invasion had become almost inevitable. If we leave U.S. missiles in Turkey, the Soviets might attack Turkey. If the Soviets do attack the Turks, we must respond in the NATO area."

That same day, however, Kennedy also authorized actions designed to send peaceful signals to Khrushchev, who had started the Cuban crisis as an obvious gamble and was signaling that he wanted to end it without fighting. Some scholars have concluded that both Khrushchev's decision to put the missiles in Cuba and his desire to get out of the ensuing crisis had provoked opposition inside the ruling Soviet politburo.

Kennedy ordered the immediate disarming of the 15 U.S. Jupiter nuclear-warhead missiles in Turkey, a step designed to show the Soviets that the United States would not use those missiles (each with enough range to reach many Soviet cities) and, at the same time, to prevent their unintended use should the Soviets attempt to attack and seize them.

Under the NATO treaty, a Soviet attack on Turkey would have led to "general war," according to Bromley Smith, who at the time was executive director of the National Security Council.

Kennedy also delayed retaliation for the downing of the U2, despite his previous order that the United States would destroy any Cuban-based antiaircraft battery that hit a U.S. plane.

Finally, Kennedy authorized his brother Robert to give his private assurance to the Soviet Union -- through its ambassador in Washington, Anatoliy Dobrynin -- that if Khrushchev took the missiles out of Cuba, the United States would respond by removing its missiles from Turkey, but that a commitment was needed the next day. The newly available notes of the Oct. 27 meeting provide the first official confirmation that a deal of this type was offered to Moscow.

Some top officials objected to that deal, but Vice President Lyndon B. Johnson asked (according to the notes of the meeting): "Why we are not prepared to trade the withdrawal of U.S. missiles from Turkey for the withdrawal of the Soviet missiles from Cuba, if we were prepared to give up the use of U.S. missiles in Turkey?"

Khrushchev accepted Kennedy's proposal; the planned U.S. attacks on Cuba did not take place. Implementation of the U.S. side of the bargain depended on the Soviets "remaining silent on the deal," according to McGeorge Bundy, who was Kennedy's national security affairs adviser and a participant in the discussions. "They kept quiet, and the missiles came out," Bundy said during a recent interview, the first time he has spoken publicly about the deal that was struck to end the crisis. Brinksmanship Lessons

The Cuban missile crisis brought the superpowers closer to nuclear war than at any time in the 40 years since Hiroshima. In drawing back from the edge, Washington and Moscow learned lessons that appear to have governed their military behavior since:

Neither side will allow the other to have an obvious nuclear advantage for very long.

In the 1950s, the United States under President Dwight D. Eisenhower not only expanded its lead over the Soviet Union in numbers of nuclear weapons but began deploying U.S. intermediate-range nuclear missiles in NATO countries. Jupiter missiles based in Turkey across from the Soviet Union were considered by the Soviets as first-strike weapons just minutes away from key Soviet cities against which no defense was possible.

U.S. officials had sent the missiles to Turkey as defensive weapons to deter a Soviet invasion but without much thought as to how the Soviets would perceive them. In the midst of the Cuban crisis, however, the Soviet point of view was recognized. In a White House meeting on Oct. 16, 1962, Gen. Maxwell D. Taylor, then-chairman of the Joint Chiefs of Staff, described the U.S. missiles in Turkey as a "pistol pointed at the head" of the Soviet Union, which would be imitated if Khrushchev succeeded in putting his missiles in Cuba, which may well have been why Khrushchev wanted to put them there.

The Cuban missile crisis gave both sides an opportunity to look seriously at the consequences of launching even some of the limited nuclear forces available at that time. What they saw convinced each side that any superpower nuclear exchange had to be avoided.

McNamara said recently that one Soviet missile in 1962 "directed at Miami or New York or even Washington, might have killed a million or 2 million people. That was something that a responsible president didn't wish to expose his nation to and was determined not to do."

Any direct military confrontation between the superpowers can quickly escalate to a nuclear showdown.

That is what happened in Cuba, and it sobered both sides. Since the 1962 missile crisis the superpowers have repeatedly taken steps to avoid situations that could lead to American and Soviet military forces confronting each other, even as the two nations have continued to compete, politically and militarily.

In those areas where U.S. and Soviet forces both operate, private agreements are worked out to prevent any military attacks on each other. When they do occur, systems exist for preventing them from getting out of hand. For example, the two nations' navies have worked out rules of the road so their sea forces that exercise near each other can avoid incidents.

So the Cuban crisis was a turning point in the nuclear age. It also provides the only detailed case of the type of crisis that, many fear, might lead to a nuclear war. Newly discovered documents and interviews with participants have provided important details on the worst -- and last -- case of a major, direct nuclear confrontation between Moscow and Washington. Seeking a Nuclear Balance

Kennedy took office in 1961 determined to correct the "missile gap" between the United States and the Soviet Union.

As McNamara said recently, however, after the Eisenhower military buildup "there wasn't any missile gap . . . rather there was a gap but it was the reverse of what had been thought." The United States had 6,000 nuclear weapons, roughly 5,500 more weapons than the Soviet Union.

Khrushchev and his military leaders were as determined as Stalin had been 15 years earlier to reach a nuclear balance between the Soviet Union and the United States. After Khrushchev's June 1961 meeting with Kennedy in Vienna -- a meeting that apparently convinced Khrushchev that the young American president could be pushed around -- the Soviet leader decided to move more quickly.

"After the Vienna meeting," Arkady Shevchenko, the highest ranking Soviet diplomat to defect to the United States, said in a recent interview, "I heard Khrushchev himself telling that Kennedy is a weak president. He is not a strong man . . . . He will not dare to do something . . . to stop what the Soviets are doing."

Shevchenko said it was cheaper and easier for the Soviets to deploy medium-range SS4 missiles in Cuba than to build and deploy new intercontinental-range missiles inside the Soviet Union.

At the time of the missile crisis, the United States had 129 intercontinental ballistic missiles (ICBMs) based on American soil; 144 missiles on Polaris submarines, and 1,300 strategic bombers. The Soviets had 44 ICBMs; 97 missiles on submarines and 155 strategic bombers, according to recently published documents.

One month before the United States determined that the Soviets had installed missiles in Cuba, a Central Intelligence Agency estimate concluded that the Soviets would not undertake such an action because it was too great a risk. Then-CIA Director John McCone, however, disagreed with his agency's conclusion and told the president that he believed Khrushchev had been strengthening Fidel Castro's aircraft and land defenses so much that it meant he would bring in missiles.

The first U2 photos clearly showing the start of Soviet medium-range missile installations in Cuba appeared the morning of Oct. 16, 1962.

Kennedy quickly called a meeting of his top aides, including Secretary of State Dean Rusk, Secretary of the Treasury C. Douglas Dillon, Attorney General Robert F. Kennedy, Vice President Johnson, McNamara, Bundy, Taylor and Bromley Smith of the NSC staff as note-taker.

This group, the Excom, began the first of a series of sessions to deal with the crisis. The first day's meetings on Oct. 16 were secretly taped by Kennedy. The transcript of the discussions that day -- more than three hours long -- was recently made public with some deletions for national security reasons. It has never before been publicized.

It illustrates more than any single document the questions, doubts and conflicting ideas that run through the minds of public leaders in the nuclear age at a time of crisis.

The first option seized upon was a surgical air strike to destroy the Soviet missiles in Cuba.

Defense Secretary McNamara stressed initially that analysts had to find and target the nuclear storage sites and that "if we are to conduct an air strike against these installations, or against any part of Cuba, we must agree now that we will schedule that prior to the time these missile sites become operational . . . because if they become operational before the air strike, I do not believe we can state we can knock them out before they can be launched."

Gen. Taylor responded that it would be difficult to determine when the missiles were operational. Speaking for the joint chiefs, Taylor said his approach would be to have "an initial pause," to get the target picture correctly, while maintaining secrecy over the fact that the United States knew the missiles were there. Then, "virtually concurrently, an air strike against the sites that we know of. At the same time, naval blockade."

These would be accompanied by reinforcement of the U.S. Navy base at Guantanamo Bay in Cuba and evacuation of U.S. dependents there.

Taylor also wanted mobilization of reserve military units but, as far as invading the island, he warned "that's the hardest question militarily in the whole business -- one which we should look at very closely before we get our feet in that deep mud in Cuba."

The idea of a blockade, which became the first option, was not put forward strongly by McNamara until the end of the first day. That may have been because when the idea was first offered by Taylor, President Kennedy said, "I don't see how we could prevent further missiles from coming in by submarine . . . . I mean if we let 'em blockade the thing, they come in by submarine." In fact, large missiles could not be transported by sub, but no one in the meeting brought that technical detail to the president's attention.

Secretary of State Rusk, not surprisingly stressed the international implications of a surgical strike. "There is no such thing, I think, as unilateral action by the United States . . . any action we take will greatly increase the risks of direct action involving our other alliances and our other forces in other parts of the world."

He went on to offer, as a first suggestion, that the United States publicly announce the existence of the missiles "some time this week" and build up forces to "deliver an overwhelming strike at any of these installations." In the interim Rusk wanted "to alert our allies and Mr. Khrushchev that there is utterly serious crisis in the making here and that Mr. Khrushchev may not himself really understand that or believe that at this point."

"I think," Rusk added, "we'll be facing a situation that could well lead to general war" and that everything ought to be done to prevent that before the positions of the two superpowers got too firm.

By the time the top officials had made their presentations, it was clear there were some basic disagreements.

One major difference among Kennedy's advisers appeared when Bundy asked "how gravely does this change the strategic balance?"

McNamara responded that the joint chiefs "said substantially," and then added: "My own personal view is, not at all."

Kennedy then noted that Khrushchev was undertaking the deployment in the face of Kennedy's own statement just a few months earlier that the United States would oppose such a move.

"They've got enough to blow us up now anyway," the president said. "I think it's just a question of . . . this is a political struggle as much as military."

Attorney General Robert Kennedy, who later in the week became a forceful advocate for an initial blockade of Cuba, on the first day supported an air strike or even invasion.

At one point he asked, "Could you stick planes over the missiles in Cuba until you made the announcement . . . to make sure that they weren't taking any action or movement?" Gen. Taylor quickly responded, "I can't visualize doing it successfully that way."

Another early Robert Kennedy suggestion was whether there was some way to create an incident in Cuba to permit military action "through Guantanamo Bay or something or whether there's some ship that, you know, sink the Maine again or something" in reference to the action that led to the Spanish American war.

Several times during the day, President Kennedy questioned his aides on why Khrushchev was trying to do something that clearly could lead to nuclear war.

At one point Taylor suggested that Cuba "makes the launching base for short-range missiles against the United States to supplement their rather decrepit ICBM system, for example."

At another point, when discussion turned to what Robert Kennedy should say to Ambassador Dobrynin about the missiles, the president mused: "I don't know whether Khrushchev is aware of what I said . . . . I can't understand their viewpoint, if they're aware of what we said . . . . I don't think there's any record of the Soviets ever making this direct a challenge, ever, really . . . since the Berlin blockade."

National security adviser Bundy attempted to soothe him by saying that "they made this decision, in all probability, before you made your statements," to which Treasury Secretary Dillon coldy added, they "didn't change it" after Kennedy's press conference.

Rusk said CIA Director McCone had "suggested some weeks ago that one thing Mr. Khrushchev may have in mind is that he knows we have a substantial nuclear superiority but he also knows that we don't really live under fear of his nuclear weapons to the extent that . . . he has to live under fear of ours. Also that we have nuclear weapons nearby, in Turkey and places like that."

Rusk went on that "Khrushchev may feel that it's important for us to learn about living under medium-range missiles, and he's doing that to sort of balance that."

At the afternoon session, when Bundy agreed with a State Department idea that perhaps Khrushchev might be putting the missiles in Cuba as a ploy to later trade for "something in Berlin, saying he'll disarm Cuba . . . if we yield some of our interests in Berlin," President Kennedy burst out saying: "It's just as if we suddenly began to put a major number of MRBMs medium-range ballistic missiles in Turkey. Now that'd be goddam dangerous, I would think."

To which Bundy replied, "Well we did, Mr. President," and Kennedy responded, "Yeah, but that was five years ago." In fact, installation of the U.S. Jupiters in Turkey began in 1960, and was continuing at the time of the Cuban crisis.

By the end of the day, the president listed three options: "We're going to take out these, uh, missiles" but questions remained as to whether there would be "a general air strike" and "general invasion."

As the week went on, however, the notes show that the Excom let the air strike, which Kennedy tentatively set for Oct. 20, slip, and moved up the blockade, which was publicly announced Oct. 22, 1962, along with disclosure of the Soviet missiles in Cuba.

As the crisis unfolded over the next five days, McCone told the Excom that the first missiles were operational; the president and his advisers weighed which Soviet ships to stop and which to let through the naval blockade.

In the end, it was determined that a military confrontation with the Soviets would be better in Cuba than on the high seas. But when Khrushchev accepted the deal Kennedy proffered -- dismantling his missiles in return for the withdrawal of U.S. Jupiters from Turkey -- no confrontation was required.

In retrospect, McNamara and Bundy said recently, it was secrecy that made possible the final resolution: secrecy allowed days of discussion and analysis, and then a back-channel offer of a confidential deal to end the crisis.

"We avoided tremendous brouhaha of selling out our European friends" by withdrawing the supposedly Turkish-owned Jupiter missiles, Bundy said recently, "but we did it keeping it secret. Not altogether a happy thing to do. It has costs, playing secret diplomacy."

McNamara observed in a recent interview that in today's world, it would be difficult to maintain the kind of secrecy that worked so well in 1962, when the world really did go to the nuclear brink.