Short answers, pulled from the story.
Walter Ulbricht sought to build a wall because 2.5 million people had fled from East Germany to West Berlin since 1949, creating a devastating brain drain that threatened the stability of the socialist government.
The border to West Berlin was shut completely on Sunday, August 13, after leaders signed an order to close the boundary at midnight on the 12th of August 1961.
Ten M48 tanks with bulldozer blades moved close to Checkpoint Charlie alongside American jeeps while thirty-three T-54 Soviet tanks drove to the Brandenburg Gate, resulting in live munitions being loaded for hours until both sides withdrew peacefully.
John F. Kennedy left the June 1961 summit feeling beaten, shaken, and angry after enduring a relentless verbal assault from Nikita Khrushchev, which set the stage for physical division of the city.
Nikita Khrushchev and John F. Kennedy agreed to withdraw tanks through backchannels managed by GRU spy Georgi Bolshakov, while West Berlin Mayor Willy Brandt and Chancellor Konrad Adenauer led the German population response.