Answer:
The letters that you are referring to are the letters that Soviet Premier Nikita Khrushchev sent to U.S. President John F. Kennedy during the Cuban Missile Crisis in October 1962.
On October 26th, 1962, Khrushchev sent a letter to Kennedy in which he offered to remove Soviet missiles from Cuba in exchange for a U.S. guarantee not to invade Cuba and to remove U.S. missiles from Turkey. This letter was seen as a hopeful sign of a peaceful resolution to the crisis.
However, the next day, on October 27th, 1962, Khrushchev sent a second letter to Kennedy that took a much tougher tone. In this letter, Khrushchev criticized Kennedy's speech the previous day, in which he had announced a naval blockade of Cuba and warned of the possibility of military action if the Soviets did not remove their missiles. Khrushchev accused the U.S. of provoking a war and threatened to take action if the U.S. did not back down.
The difference between the two letters is significant. The first letter was conciliatory and offered a potential path to resolving the crisis peacefully, while the second letter was confrontational and increased tensions between the two sides. The second letter showed that Khrushchev was willing to take a more aggressive stance in response to Kennedy's actions, which made it clear that the situation was still very dangerous and a peaceful resolution was not guaranteed.