The roots of the 1961 Berlin Crisis lie in the situation following World War II. When the war ended in 1945, the allied forces separated Germany into four zones and Berlin into two parts. The USA, along with the UK and France, influenced the western part. And the Soviet Union controlled the easter part. Both sides kept a military presence in their respective zones.
However, the Soviets hoped they would be able to control the whole of Berlin. In 1948, they blocked all entrance roads to the city. Thus, the USA was unable to access the city. As a result, they transported cargo to Berlin via airplanes. The Berlin blockade (or the First Berlin Crisis) and airlift lasted for over a year.
In November 1958, the Soviet leader Nikita Khrushchev demanded that the Western powers (USA, UK, France) withdraw from Berlin. He warned to abandon the agreement, which divided the city into two parts. The West refused.
Nikita Khrushchev met with newly elected US president John F. Kennedy on June 4th, 1961, in Vienna. During the summit, he once again threatened the USA with the same issue. Once again, Kennedy refused as Berlin was a strategic point. Besides, thousands of refugees from the Soviet Block used Berlin to flee the communist terrors.
During the night from August 13th to 14th, the Soviets once again sealed entrances to Western Berlin. More importantly, they began to erect a concrete wired barrier – later known as The Berlin Wall. They built the wall on their territory, hence, the West could not intervene. Escaping to the West was now almost impossible.
The West and the particularly Soviets continued with provocation. In the late hours of October 27th, 1961, an armed conflict was imminent. Tanks of the two powers faced each other, separated by only a few meters. Neither party fired in the stand-off.
Thus, the post-war status quo lasted until 1989. Then, Berlin unified, and the wall was torn down. The Berlin Crisis of 1961 was one of the most heated events of the Cold War. Closer to an actual war was only the Cuban Missile Crisis a year later.