Berlin’s location was a problem for the Western powers because it was deep inside the Soviet zone of occupation, which later became East Germany.
At the end of World War II, the Allies divided Germany up into four zones of occupation, each to be run by one power. They also divided Berlin into four zones. The Western Allies soon consolidated their three zones into one zone. Their part of Berlin came to be called West Berlin and their part of Germany came to be called West Germany.
It was important for all four parties to hold part of Berlin. This was because Berlin was the symbolic heart of Germany. If either side had held Berlin exclusively, it would have had more of a moral and psychological claim to be in control of the “real” Germany.
This is why Berlin was a problem for the West. The West needed to maintain a major presence in West Berlin in order to retain its ability to claim that it held the “real” Germany. However, Berlin was deep inside enemy territory, with no land access except through that territory. This meant that Berlin could easily become a focal point for conflict and crisis, as it did in both 1948 and 1961.
In July and August of 1945, the Potsdam Agreement called for the division of post-war Germany into four temporary zones controlled by the U.S., England, France, and Russia. The city of Berlin was also divided into these four zones; however, the problem was that Berlin was located about 100 miles within the Soviet-controlled zone.
The Soviets established Communist control in their zone, and the Western powers believed that the Soviets planned a complete takeover of Germany after the other powers had withdrawn. There was also no clear agreement on the control of shipping to West Berlin from the American, French, and English zones of West Germany. The Soviets only allowed one cargo route with a limit of ten trains per day and did not expand this access, and they also offered limited air corridors to West Berlin.
In June of 1948, the Soviets cut off all rail, road, and water access to West Berlin from West Germany after the Western powers had introduced a new German currency that the Soviets opposed. The U.S. and Great Britain launched the Berlin Air Lift to provide food and supplies to West Berlin. A cargo plane landed every 45 seconds at Tempelhof Airport at the height of the airlift to supply West Berlin with enough food for its people to survive, and the Soviets ended the blockade on land access to West Berlin in May of 1949.