President Kennedy's moderate response during the Cuban missile crisis was perceived instead as "a virtual declaration of war" by the Soviets, and Premier Nikita Khrushchev backed down from the brink for unexpected reasons, scholars now say.

Newly declassified documents indicate that luck, as well as skill, played a great role in resolving the world's last major nuclear crisis 27 years ago, Soviet and American experts at Harvard University said Wednesday.Kennedy and Khrushchev made key decisions based on serious misperceptions and faulty intelligence, which, however, may have helped to resolve the crisis as well as to start it, said Graham T. Allison, dean of Harvard's John F. Kennedy School of Government.

"Looking at it in retrospect, with all the new information, I still give high marks to President Kennedy and . . . also, perhaps surprisingly, to Khrushchev, for the degree of control they tried to exercise," said Allison, author of "Essence of Decision," one of the authoritative books on the crisis.

"But nonetheless, it is now clear that there was a great component of good fortune. It was a combination of extraordinary skill, extraordinary courage and equally extraordinary good luck that permitted us to avoid catastrophe," he said.

Harvard researcher James T. Blight said the documents, examined during two recent international conferences, demonstrate that the Soviets knew about U.S. covert operations to destabilize the Cuban regime.

The Soviets had put the missiles in Cuba mainly because they believed a full-scale Marine invasion was likely, said Blight, co-author of a new book, "On The Brink: Americans and Soviets Reexamine the Cuban Missile Crisis."

Kennedy and his advisers believed that setting up a naval blockade was "a relatively mild response" to the missiles that would help defuse the crisis by rendering an invasion unnecessary.

"They had no idea that the way that blockade would be perceived in Moscow was as a prelude to an invasion and a virtual declaration of war," Blight said.

Indeed, the documents reveal that an enraged Nikita Khrushchev ordered Soviet ships to steam through the U.S. blockade, but the order was reversed by Anastas Mikoyan, the first deputy Soviet premier, just hours before a military confrontation would have occurred.

It remains unclear whether Mikoyan circumvented the decision on his own or obtained Khrushchev's permission to change it.

Allison said it now appears that Khrushchev gave in partly because he had received an urgent message from Castro, dictated from a bunker, saying an American military attack was imminent.

A Soviet general, Dmitri A. Volkogonov, also advised that an attack was coming, which turned out not to be true, said Allison.

The crisis flared on Oct. 15, 1962, when a high-flying American U-2 spy plane photographed the installation of nuclear-tipped Soviet SS-4 and SS-5 missiles in Cuba. Kennedy deliberated secretly with his aides for a week before announcing a naval quarantine of Cuba and a demand that the Soviets withdraw the missiles.