During World War II, the United States fought together with Britain and the Soviet Union to defeat of Nazi Germany. The Soviet Union used the defeat of the Nazis as an opportunity to expand its territory, pushing its boundary much farther west and taking effective control of a number of Central European nations, including a section of Germany itself. The United States and Britain woke up to a new reality; they had stopped Germany from conquering Europe, but now most of Europe was in the hands of the Soviet Union. When the Soviet Union sought to take control of the sectors of Berlin still under the control of the United States and its allies, the U.S. decided to push back and try to stop the expansion of the Soviet Union. Thus began the Berlin Airlift to supply West Berlin, which had been blockaded by the Soviet forces. The Cold War had begun.

The author of the passage above infers that the cause of the Cold War was