What is the importance of Berlin in the nonfiction book, Thirteen Days; a Memoir of the Cuban Missile Crisis?

The former capital of Germany, Berlin is located deep in the Democratic Republic of Germany after World War II and divided into four sectors by the victorious Allies. Soon the Soviets restrict access to West Berlin from West Germany, precipitating a crisis that nearly rise to military confrontation. JFK first meets Khrushchev in Vienna to discuss lessening tensions in Berlin, but comes away worried the Soviet Premier misjudges his "mettle." JFK anticipates that if he takes military action against the Soviet missiles in Cuba, the Soviets will again blockade Berlin.