Discuss the origins of the Cold War. How did the US and the Soviet Union go from World War II allies to superpower confrontation?

One major factor was the use of the atomic bomb by the United States. The United States kept the existence of the technology secret from the Soviet Union until the day it was dropped in Japan. Stalin, who had spies in the United States at the time, knew of the new technology but did not know the details. The Soviet Union rushed to create an atomic weapon after the war and the United States only knew about the nuclear test when radioactive particles were found over Soviet airspace during a spy plane flyover.
The Soviet Union also did not promote free elections in Eastern Europe despite promises at Yalta to do so. The Soviet Union jailed or murdered anyone they thought would be a threat in the future—seen, for example, in the murder of Polish officers at the Katyn Forest during World War II. The United States suspected that the Soviet Union had designs on all of Europe. When Stalin was confronted due to this, he claimed that he had a right to a sphere of influence just like the Americans had a right to the Western Hemisphere. Stalin also claimed that the Allies were slow to create a second front in France in order to bleed the Soviets of some of their power. Both the Americans and the Soviets never fully trusted each other during the war, and actions after the war only furthered that mistrust. By the end of the Truman administration, both sides refused to budge.