On October 22, Kennedy announced the discovery of Soviet missiles on television and said that he would impose a naval “quarantine” of Cuba in two days. He warned that the launch of a single missile from the island would cause “a full retaliatory response upon the Soviet Union.” This meant a massive U.S. nuclear attack on the Soviet Union and its Eastern European satellites.

Meanwhile, U.S. Air Force and CIA planes were flying high and low over Cuba to glean intelligence on the Soviet missile buildup and to prepare plans for attacking key installations and invading the island. On October 27, an American U-2 spy plane was shot down over Cuba by a Soviet surface-to-air missile (SAM). When Kennedy wondered aloud why Krushchev would do this, Defense Secretary Robert McNamara answered, “I don’t know how to interpret it.”

There is much that U.S. officials did not know how to interpret during the crisis, and much that they misinterpreted. For example, when the U-2 was shot down, all American officials assumed that the anti-air firing on U.S. planes was authorized by Khrushchev. In fact, however, Khrushchev was not in control of anti-aircraft forces in Cuba. Rather, Cuban personnel were conducting operations under the direction of Fidel Castro. Castro had feared an imminent invasion and ordered his anti-aircraft personnel to fire on American planes. Some Soviet operators were carried away by the example of their Cuban comrades and ignored orders not to fire without authorization from the Soviet general in charge in Cuba. This turned out to be the case with the Soviet officer who had successfully struck the U-2 plane.

That night, Attorney General Robert Kennedy met secretly with Soviet Ambassador Anatoly Dobrynin at the Justice Department and delivered a two-part ultimatum. The first demand was that the Soviets begin removing the missiles within 48 hours, or the U.S. would attack them. The second demand, which long remained secret, was that firing on American reconnaissance planes must end immediately. According to Ellsberg’s notes, the attorney general declared, “‘If one more plane was shot at, we wouldn’t just attack the site that had fired at it; we would take out all the SAMs and anti-aircraft and probably all the missiles. And that would almost surely be followed by an invasion.’”

In threatening invasion for actions over which Krushchev may not have had control, Kennedy ominously did not know that the Soviets had deployed 98 “tactical” nuclear weapons with Soviet forces on Cuba. These weapons were to defend Cuba against an expected marine invasion by the U.S. Prior to October 22, local officers were pre-authorized to use them. Neither the existence of these weapons nor the delegation of authority to use them was known to Americans until 30 years later.

Another unknown—or mistakenly known—“fact” also added impetus to an American invasion. The Defense Department told Kennedy that there were “about 8,000-10,000” Soviet “probably military personnel” in Cuba. Calculations of what it would take to complete an invasion followed from this. In fact, the Soviets had roughly 42,000 troops on the island, which also was not known until decades later.