Excerpted fromOne Minute to Midnight: Kennedy, Khrushchev, and Castro on the Brink of Nuclear War,by Michael Dobbs, to be published this month by Knopf; © 2008 by the author.

The Cuban missile crisis on Saturday, October 27, 1962, reached its moment of maximum peril, but John F. Kennedy was determined not to miss his regular swim. The president usually swam twice a day, [#image: /photos/54cbfac744a199085e8917ea]just before lunch and just before dinner, often with his longtime aide Dave Powers. Kennedy’s doctors had prescribed swimming for his back, but it was also a way of relaxing. Originally built for Franklin Roosevelt, the indoor pool in the West Wing basement had recently been refurbished with a mural of a sailing scene in the Virgin Islands—a gift from the president’s father.

Returning from his midday swim, Kennedy passed by the Oval Office before heading up to the residence for lunch. The phone rang at 1:45 p.m. It was Secretary of Defense Robert McNamara, and the news he reported could hardly have been worse: an American U-2 spy plane had gone missing off Alaska and may have strayed into Soviet territory. This was more than just an unfortunate incident: the intrusion into Soviet airspace by an American military plane at the height of a nuclear showdown between the two superpowers was a dangerously provocative act.

October 27 was the day that would come to be known around the White House as “Black Saturday.” Five days had gone by since Kennedy’s televised address to the nation revealing the presence of Soviet nuclear missiles in Cuba, and events were spinning out of control. Earlier that day the Soviet premier, Nikita Khrushchev, had upped the ante in the diplomatic negotiations by demanding the withdrawal of American missiles from Turkey. An American U-2 spy plane had been shot down over eastern Cuba. The island had been sealed off by an American blockade, and U.S. warships were challenging nuclear-armed Soviet submarines in the Caribbean.

A few minutes after McNamara’s call Roger Hilsman, the chief of intelligence at the State Department, came running up the stairs from the basement office of McGeorge Bundy, Kennedy’s national-security adviser. Hilsman had just learned that the Soviets had scrambled MiG fighters to intercept the missing U-2, and that the U.S. Strategic Air Command (sac) was scrambling American fighters in response. After two days without sleep, Hilsman was exhausted, but he fully understood the significance of what was happening. The Soviets might well perceive the U-2 incursion as a harbinger of an American nuclear attack.

Hilsman expected a furious outburst from the president, or at least some sign of the panic he himself was beginning to feel. Instead, Kennedy responded with a short, bitter laugh.

“There’s always some son of a bitch who doesn’t get the word.”

J.F.K.’s calm exterior belied a deep frustration. His experiences in World War II, as the skipper of a P.T. boat in the South Pacific, had taught him an abiding lesson about modern warfare: a commander in chief, however well informed, however powerful, cannot possibly exercise complete control over the battlefront. Mistakes were an inevitable consequence of warfare, but in previous wars they had been easier to rectify. The paradox of the nuclear age was that American power was greater than ever before—but it could all be jeopardized by a single, fatal miscalculation.

The historian turned Kennedy aide Arthur M. Schlesinger Jr. would later describe Black Saturday as “the most dangerous moment in human history.” The Strategic Air Command had increased its readiness level to defcon 2—one step short of nuclear war—and nearly 3,000 American nuclear warheads were targeted on the Soviet Union. Fidel Castro had gone to the Soviet Embassy in Havana to urge Khrushchev to consider using nuclear weapons to “liquidate” the imperialist enemy once and for all. Unbeknownst to Kennedy, the Soviets had dispatched nuclear warheads to two missile sites in Cuba, ready to destroy American cities. And at dawn on Saturday morning, also unknown to Washington and reported here for the first time, Soviet troops had moved nuclear-tipped cruise missiles to a “firing position” 15 miles from the U.S. naval base at Guantánamo Bay.