Final answer:
In response to Khrushchev's demand, President Kennedy called up additional troops to reinforce the U.S. commitment to West Berlin, a move that maintained the U.S. presence in the city amid escalating Cold War tensions.
Step-by-step explanation:
When Nikita Khrushchev demanded that US troops leave West Berlin, President John F. Kennedy responded by calling up additional troops to demonstrate U.S. commitment to defending the city. The tensions in Berlin escalated when Khrushchev built the Berlin Wall in 1961, further heightening Cold War tensions. However, Kennedy did not back down, hinting at the use of "tactical" nuclear weapons until Khrushchev backed away from his threats, and Kennedy's visit to Berlin in 1963 reinforced American solidarity with West Berliners through his iconic "Ich bin ein Berliner" speech.