Tensions between the US and USSR exploded in 1948, when the Soviet Union blockaded Berlin. The US responded with a yearlong airlift to supply Berliners stranded in the western zone of the city.
Realizing that conflict with Russia might escalate into war, the United States joined the North Atlantic Treaty Organization (NATO) defensive alliance and ramped up security measures at home with the National Security Act.
The Berlin airlift
The growing tensions between the United States and the Soviet Union erupted in 1948, divided by ideological divisions over what should be done with Germany.
Believing that a reunified Germany would prevent a repeat of the economic catastrophe that had followed World War I, the US, Britain, and France decided to combine their zones of Germany. The USSR objected, because it wanted to make sure that Germany could never attack it again. In 1948 the USSR cut off all highway and railroad access to Berlin (located within its occupation zone), hoping to bring the entire city under Soviet control. West Berlin would either starve or the Western Allies would surrender to the Soviets' wishes for Germany.
Instead, from June 1948 to May 1949, the US, Britain, and France sent hundreds of airplanes filled with food and fuel every day in what became known as the Berlin airlift.
After 11 months, the Soviets realized that the blockade was a failure. They ended it, but the standoff over Berlin had clarified the sides in the Cold War. The Western Allies combined their occupation zones into the new country of West Germany, and the Soviets responded by creating East Germany. The Soviets began the process of building a wall between Berlin’s eastern and western zones.
Source: Start of the Cold War—The Berlin Airlift and the Creation of NATO
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