By Jeff Franks

HAVANA (Reuters) - Fifty years ago, in October 1962, Estela Rivas Vasquez stood watch in a newly dug bunker outside Havana's grandiose Hotel Nacional, using binoculars to scan the Straits of Florida for signs of an American invasion.

She did not fully know why Cuban leader Fidel Castro had put the country on a war footing, but as a civilian militia member she was certain of one thing - at the tender age of 18 she was prepared to die for her country.

"I'll tell you the truth - I did not feel fear ... I had to defend the country and I didn't care about dying," said Rivas, a petite woman still as feisty as she was then.

U.S. President John F. Kennedy and Soviet Premier Nikita Khrushchev eventually negotiated an end to what became known as the Cuban Missile Crisis, which began on October 16, 1962 when Kennedy was told the Soviet Union had placed nuclear missiles on Cuba, barely 90 miles off the Florida coast.

On October 28, after 13 tense days with the world on the verge of nuclear war, Khrushchev agreed to withdraw the missiles in exchange for Kennedy's promise that the United States would never invade the island, with whom U.S. relations had steadily worsened after Castro took power in a 1959 revolution.

Most of the world was relieved that a potential World War III had been avoided, but in Cuba the reaction, led by Castro, was at best ambivalent. Although tempered by time, it remains so today.

BETRAYED

Cubans shared the world's relief 50 years ago, but felt betrayed because Castro had been excluded from the negotiations.

They believed his absence led to a deal that did not include an end to the U.S. trade embargo against the communist-ruled island and the return to Cuba of the U.S. naval base at Guantanamo Bay, both still key issues today.

Young people, including Rivas, took to the streets shouting chants against Khrushchev.

"We got really angry when Nikita withdrew the rockets, because they didn't consult with Fidel. We lost the opportunity to get Guantanamo back," Rivas said glumly.

When pressed on how that might have been done, she suggested that with fear of nuclear war hanging over its head and the Soviet Union backing Cuba, the U.S. might have changed its policies.

"If the Americans had said there's a Soviet base in Cuba, we'd be able to say there's an American base, too," she said.

But Kennedy and Khrushchev were less concerned with resolving Cuba's grievances than with ending the crisis as quickly as possible, said James Blight, a history professor at the University of Waterloo in Canada, who recently published "The Armageddon Letters," a book on the missile crisis.

With the fate of the world in the balance, there was no time for the dangerous gamesmanship the Cubans wanted to play.

"Khrushchev was so frightened at this point that the world is going to blow up he couldn't care less what the Cubans say. He would deal with that later," Blight said.

Out in the field, the Soviets had ordered the Cubans not to fire at low-flying U.S. reconnaissance planes buzzing over the island, fearing it could provoke an American response.

Huddled in a trench in the mountains west of Havana, militia member Orlando Iglesias watched in terror day by day as the planes roared by, barely above the treetops.

"Those were terrible moments, I thought I could die and I thought of my children, who were very young," remembered the mild-mannered Iglesias, now 86.

His fellow Cubans wanted to shoot at the planes, but they followed the Soviet orders until October 26, when an increasingly frustrated and desperate Castro, thinking a U.S. invasion was inevitable, ordered the firing ban lifted.

The next day, an American U-2 spy plane was shot down over eastern Cuba and Castro wrote a letter to Khrushchev urging him to unleash a nuclear attack on the United States if it invaded Cuba.

"That would be the moment to eliminate such danger forever through an act of clear legitimate defense, however harsh and terrible the solution would be, for there is no other," wrote Castro, who was 35 at the time.

Khrushchev viewed his suggestion as madness.

Blight said it was a reflection of a "streak of martyrdom" running through Cuban history, which he characterized as "don't give in, don't compromise, don't say yes, if you can't get the whole thing, it's better to die in the service of your nation, your people."

Today, some of the Soviet missiles, freshly painted with a red hammer-and-sickle, are displayed in a Havana park as a tourist attraction and Rivas gives tours of the Hotel Nacional, including the bunkers where she kept watch for American ships.

Castro, now 86 and out of power since 2008, would later in life become a campaigner against nuclear weapons.

(Reporting by Jeff Franks and Rosa Tania Valdes; Editing by David Adams and Paul Simao)