Quick wikipedia summary: Soon after the construction of the Berlin Wall in August 1961, a stand-off occurred between US and Soviet tanks on either side of Checkpoint Charlie.

It began on 22 October as a dispute over whether East German border guards were authorized to examine the travel documents of a US diplomat based in West Berlin named Allan Lightner heading to East Berlin to watch an opera show there, since according to the agreement between all four Allied powers occupying Germany, there was to be free movement for Allied forces in Berlin and that no German military forces from either West Germany or East Germany were to be based in the city, and moreover the Western Allies did not (initially) recognize the East German state and its right to remain in its self-declared capital of East Berlin. Instead, Allied forces only recognised the authority of the Soviets over East Berlin rather than their East German allies.

By 27 October, ten Soviet and an equal number of American tanks stood 100 yards apart on either side of the checkpoint. This stand-off ended peacefully on 28 October following a US-Soviet understanding to withdraw tanks and reduce tensions. Discussions between US Attorney General Robert F. Kennedy and KGB[citation needed] spy Georgi Bolshakov played a vital role in realizing this tacit agreement.