There is no specific date that marked the beginning of the Cold War, but most agree that it began shortly following the second World War, somewhere from 1945 to 1947. Unlike other wars, the Cold War never involved actual fighting or traditional war events, but instead describes a prolonged tense state between the United States and Russia.
Russia and the United States were both members of the Allied forces that fought against the Axis powers of World War II. However, due to extreme differences between the cultures, the relationship between the two allies was strained. Following the war, an arms race began between the two countries as a result of new nuclear weapon technology.