Fifty years ago, the discovery of Soviet nuclear missiles in Cuba set off the most acute crisis of the Cold War, and possibly the most dangerous moment in human history.

Afterward, men on both sides of the drama came away believing only luck prevented the two superpowers from plunging the world into a nuclear conflagration.

Over the decades, the missile crisis has been portrayed as a masterful performance by president John F. Kennedy, with his admirers recounting how he kept his nerve and averted war.

His example is often held up as a model of leadership under pressure, and US Secretary of State Hillary Clinton has compared the current administration's approach to Iran's nuclear program as similar to Kennedy's "high-stakes diplomacy."

But documents from previously secret Soviet and US archives have revealed a more prosaic reality: that during those 13 days in October 1962, Kennedy and his Soviet counterpart, Nikita Khrushchev, often struggled to control an escalating chain of events.

Concerned about the Americans' edge in nuclear weapons and US efforts to topple Cuba's pro-Moscow regime, Khrushchev decided in May 1962 to send more than 40,000 troops and dozens of nuclear missiles to Cuba.

All the while, Khrushchev kept reassuring Washington that deploying Soviet offensive weapons to Cuba was out of the question.

American leaders were astonished when they learned on October 16 of the presence of Soviet missiles in Cuba, captured in photos taken by a U-2 spy plane.

"The dominant feeling was one of shocked incredulity," the president's brother, Robert Kennedy, later recounted.

The strategic surprise was total. US spy agencies had failed to spot the warning signs.

There had been numerous CIA reports from Cuban informers about suspicious convoys by night, but the intelligence service distrusted the vague accounts, which contradicted a strongly-held assumption that Moscow would not dare to deploy the bomb so close to America, according to Michael Dobbs' history of the crisis, "One Minute to Midnight."

US missiles in Turkey

At the White House, top generals recommended air strikes followed by a possible invasion of Cuba, while the defense secretary Robert McNamara and senior diplomats preferred a blockade of the island to prevent Soviet naval ships from ferrying in more weapons.

On October 22, Kennedy laid out the crisis in a speech to Americans and ordered US forces on a state of maximum alert. The president rejected the advice of his generals and opted for the naval blockade.

With US warships in place, the White House waited anxiously for the arrival of Soviet ships steaming towards Cuba. The Soviet vessels turned around and headed back home, a move greeted with relief around the world.

Behind the scenes, however, tensions rose.

Kennedy and Khrushchev searched for a way out of the deadlock, but their efforts were hamstrung by crossed wires, misunderstandings and cumbersome diplomatic correspondence that allowed for no direct channel of communication.

On the evening of October 26, the Soviets proposed withdrawing their missiles on the condition the Americans promise not to invade Cuba. But the following day, Moscow issued a public demand for the United States to pull out its Jupiter missiles from Turkey.

On October 27, what would become known as "Black Saturday," a U-2 aircraft was shot down over Cuba and Kennedy's advisers then debated retaliatory air strikes. Events appeared to be spinning out of control.

The Pentagon was poised to carry out a massive bombing on Tuesday, followed by an all-out invasion of 120,000 troops -- an operation comparable to the D-Day landings in World War II.

It was not until 30 years later that the Americans learned "the Soviets had dozens of short-range battlefield missiles on the island, equipped with nuclear warheads capable of wiping out an entire invading force," wrote Dobbs.

Just as the crisis reached a crescendo, the two sides clinched a deal.

Under a compromise, Washington pledged not to invade Cuba and secretly agreed to pull out its missiles from Turkey, while Moscow promised to withdraw its warheads from the island.

"For many years, I considered the Cuban missile crisis to be the best managed foreign policy crisis of the last half-century...," the then Pentagon chief McNamara told a conference in Havana in 2002.

"But I now conclude that, however astutely the crisis may have been managed, by the end of those extraordinary 13 days, luck also played a significant role in the avoidance of a nuclear war by a hair's breadth."

For the former head of the KGB's Cuban department, Nikolai Leonov, the peaceful ending seemed like a miracle. "It is almost as if some divine intervention occurred to help us save ourselves."