Answer:
Stalin blockaded Berlin for a number of strategic reasons. Before we delve into why he blockaded Berlin, we must first know a couple background information.
The Berlin Blockade (which led to the Berlin Airlift), was an event that occurred following World War II. The Allies decided to draw out and divide the previous Nazi Germany between them, as well as the Capitol, Berlin. Each of the 4 countries (USSR, US, UK, and France) were to receive 1/4 of Germany as well as 1/4 of Berlin, the capitol (though it was located in the heart of Russian territory).
In the following years, the USSR decided to try to wrest control of Germany as a whole from their Western "allies", and decided to start by choking the western half of Berlin, officially starting the Berlin Blockade. In this Blockade, he cut off all supplies and connection from Berlin to Western Germany, as well as creating a wall called the Berlin Wall to further segregate the Berliners. He also posted numerous troops in an attempt to stop NATO supply shipments. This was brought to the US, who started the Berlin Airlift, or a series of aerial supply drops on Berlin that brought them food, medical supplies, etc. etc. The dedication from the Allies soon proved that the Blockade was not worth the resources, so Stalin soon lifted the blockade, admitting defeat.
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