The president of the United States during the Cuban Missile Crisis was John F. Kennedy.

The Cuban Missile Crisis was a major confrontation between the United States and the Soviet Union that occurred in October 1962. It was a tense period of about two weeks when the world stood on the brink of nuclear war.

President John F. Kennedy played a crucial role in navigating this crisis and is often credited with averting a potential catastrophe.

Here is a detailed explanation of why the correct answer is John F. Kennedy:

I. Background of the Cuban Missile Crisis:
- In 1959, Fidel Castro led a revolution in Cuba and established a communist government.
- This alarmed the United States, as it was concerned about the spread of communism in its sphere of influence.
- The United States attempted to undermine Castro's regime through various means, including economic sanctions and support for anti-Castro forces.

II. Soviet Union's Response:
- In response to the United States' actions, the Soviet Union, under the leadership of Nikita Khrushchev, sought to strengthen its influence in the Western Hemisphere.
- Khrushchev decided to deploy nuclear missiles in Cuba, which would have given the Soviet Union the capability to strike the United States directly.

III. Discovery of Soviet Missiles:
- In October 1962, U.S. intelligence discovered evidence of Soviet missile installations in Cuba.
- This discovery set off alarm bells in Washington, as it meant that the United States was now within range of Soviet nuclear weapons.

IV. President Kennedy's Response:
- Facing this grave threat, President Kennedy assembled a group of advisors known as the Executive Committee of the National Security Council (ExComm).
- Kennedy's ExComm debated various options, including a military strike on the missile sites or a naval blockade of Cuba.
- Ultimately, Kennedy decided to impose a naval quarantine, or blockade, around Cuba to prevent further shipments of Soviet missiles.

V. Diplomatic Negotiations:
- Kennedy also pursued diplomatic negotiations with the Soviet Union to resolve the crisis peacefully.
- Through backchannel communications with Khrushchev, Kennedy proposed a deal in which the United States would remove its missiles from Turkey in exchange for the Soviet Union removing its missiles from Cuba.

VI. Resolution of the Crisis:
- After tense negotiations, Khrushchev agreed to Kennedy's proposal, and the crisis was resolved without further escalation.
- The United States and the Soviet Union reached a secret agreement, known as the Kennedy-Khrushchev understanding, to end the crisis.

In conclusion, John F. Kennedy was the president of the United States during the Cuban Missile Crisis. His leadership and decision-making during this crisis were instrumental in preventing a nuclear war between the United States and the Soviet Union.