The four powers governing Berlin ( Soviet Union , United States , United Kingdom , and France ) had agreed at the 1945 Potsdam Conference that Allied personnel could move freely in any sector of Berlin . But on 22 October 1961 , just two months after the construction of the Wall , the US Chief of Mission in West Berlin , E. Allan Lightner , was stopped in his car ( which had occupation forces license plates ) while crossing at Checkpoint Charlie to go to a theatre in East Berlin . The former Army General Lucius D. Clay , US President John F. Kennedy 's Special Advisor in West Berlin , decided to demonstrate American resolve . Clay sent an American diplomat , Albert Hemsing , to probe the border . While probing in a vehicle clearly identified as belonging to a member of the US Mission in Berlin , Hemsing was stopped by East German police asking to see his passport . Once his identity became clear , US Military Police were rushed in . The Military Police escorted the diplomatic car as it drove into East Berlin and the shocked GDR police got out of the way . The car continued and the soldiers returned to West Berlin . A British diplomat — British cars were not immediately recognisable as belonging to the staff in Berlin — was stopped the next day and showed his identity card identifying him as a member of the British Military Government in Berlin , infuriating Clay
Potsdam Conference
Potsdam Conference
Berlin Crisis of 1961