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In 1945, just after World War II, the alliance between the United States, Britain and the Soviet Union ended. An intense rivalry between communist and non-communist nations led to the Cold War. It's called the Cold War because it never led to armed or "hot" conflict. At the end of World War II, at the Yalta Conference, Germany was divided into four occupied zones controlled by Great Britain, France, the Soviet Union, and the United States. Berlin was also divided into four sections. Lack of a mutual agreement on German re-unification was a important background of the Cold War. And on March 5, 1946, Winston Churchill, gave his "iron curtain" speech while at Westminster College in Fulton, Missouri, which marked the start of the Cold War.
The cold war did not end until the collapse of the Soviet Union in 1991. During this period, the United States and the USSR confronted each other in politics, economy, ideology, and so on. And they nearly divided this world into two camps, socialist camp and capitalist camp, what made the conflict on ideology especially sharp.
Every incident in the world could not happened without reasons, and the original cause may happened quite long ago. So there are long term causes and short causes of the Cold War. One of the short term causes is that the US President had a personal dislike of the Soviet leader Josef Stalin. At the Potsdam Conference starting in late July 1945, serious differences emerged over the future development of Germany and Eastern Europe. At Potsdam, the US was represented by a new president, Harry S. Truman, who on April 12 succeeded to the office upon Roosevelt's death. Truman was unaware of Roosevelt's plans for post-war engagement with the Soviet Union, and more generally uninformed about foreign policy and military matters. The new president, therefore, was initially reliant on a set of advisers. This group