Short answers, pulled from the story.
The Berlin Crisis of 1961 was triggered by two pressures converging: the mass flight of East Germans to the West and Soviet Premier Nikita Khrushchev's determination to force the Western allies out of Berlin. Since 1949, 2.5 million people had left East Germany, destabilising the economy and prompting East German leader Walter Ulbricht to seek Soviet backing for a forceful solution.
On the 27th of October 1961, American and Soviet tanks faced each other at Checkpoint Charlie, loaded with live munitions, just 50 to 100 metres apart for roughly eighteen hours. The standoff was resolved when Kennedy offered to ease pressure over Berlin in exchange for the Soviets withdrawing their tanks first; a Soviet tank moved back about five metres first, then the Americans followed.
Khrushchev issued his first Berlin ultimatum in November 1958 to force the Western allies to sign a peace treaty recognising East Germany as an independent state and to withdraw their troops from Berlin. He gave the allies a six months deadline, threatening to sign a separate peace with East Germany and declare Berlin a free city if they refused, which would have given East Germany control over all access routes.
Yes. On the 6th of August 1961, a functionary inside the Socialist Unity Party provided the 513th Military Intelligence Group in Berlin with the correct construction start date. At a Watch Committee meeting on the 9th of August, the Chief of the U.S. Military Liaison Mission predicted a wall would be built, and an intercepted SED communication the same day confirmed plans to close the border.
Kennedy privately described the Wall as "not a very nice solution, but a wall is a hell of a lot better than a war." He chose not to respond militarily to the Wall's construction, instead sending Vice President Lyndon B. Johnson and General Lucius D. Clay to West Berlin on the 19th of August 1961 to demonstrate American solidarity with the city's population.
Yes. According to Brigadier General Alvin Cowan, speaking at the Tactical Nuclear Weapons Symposium of 1969, the United States deployed the Davy Crockett tactical nuclear recoilless gun during the Berlin Crisis of 1961. Cowan noted the weapon was later retired partly because of "great fear that some sergeant would start a nuclear war."