 Complete the following phrase:  East German leader Walter Ulbricht agitated and maneuvered to get the Soviet Unions permission to construct the Berlin Wall in 1961 to stop emigration and defection westward through the Border system, preventing escape across the city sector border from East Berlin into West Berlin. Checkpoint Charlie became a symbol of the Cold War, representing the separation of East and West. Soviet and American tanks briefly faced each other at the location during the 