Which event most directly led to construction of the Berlin Wall?

At the 1961 Vienna Summit, Soviet leader Nikita Khrushchev renewed pressure over Berlin, but negotiations with President Kennedy did not resolve the crisis. As large numbers of East Germans continued fleeing to West Berlin, East Germany built the Berlin Wall to stop them. The 1948 blockade and airlift happened much earlier, and the Bay of Pigs invasion was not the direct cause.

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