At the end of World War II, the Allies had reached an agreement about what the world would like in post-war Europe. The Soviet Union chose not to follow many of these agreements leading to the start of the Cold War.
One agreement that was made was about how the post-war government in Poland would become composed. It was agreed there would be free elections, and the new government would have some members of the pre-war government in it. However, the new government had very members of the pre-war government in it, and it appeared there were no free elections occurring.
Another agreement made was reflected in the Declaration of Liberated Europe. This agreement said countries would be free to choose the kind of government they would like to have. However, the King of Romania said he was pressured by the Soviet Union into having a communist government.
The Soviet Union tried to spread communism throughout Europe and Asia. We developed a policy designed to stop the spread of communism known as containment. Thus, we helped European countries with the creation of the European Recovery Program. This program offered aid to countries resisting the spread of communism. We helped West Berlin when the Soviet Union cut off the land routes to West Berlin in an attempt to turn West Berlin into a communist area. We developed the Berlin Airlift to fly supplies over the blockade. We got the United Nations to help South Korea when the Soviet-supported North Korea invaded South Korea to try to unite Korea into a communist country.
We responded to the breaking of agreements and the aggressive actions of the Soviet Union after World War II ended. A Cold War is a period of confrontation and competition between countries. The actions of the Soviet Union led to the beginning of the Cold War. This lasted until about 1990.