The Cold War began in 1947, when the brief alliance between the United States of America and the Soviet Union which had subsisted during the Second World War broke down, leaving two power blocs covering most of the Northern hemisphere. The United States controlled the Western Bloc, while the Soviet Union dominated the Eastern Bloc. The term "Cold War" refers to the tension between these two blocs, in which the United States and the Soviet Union built up formidable arsenals of both nuclear and conventional weapons. Because each side was so powerful, it was generally believed that they could not engage directly with one another for fear of bringing about the apocalypse. The Cold War ended with the disintegration of the Soviet Union in 1991.

The above summary is precisely 125 words. Of course, this means that it has to leave out a great deal of information, even on a fairly basic level. Perhaps the most significant matter not covered is the way in which the Western and Eastern Blocs did engage in aggression without direct conflict between the United States and the Soviet Union. This occurred through the fighting of proxy wars, most notably in Korea and Vietnam, and extensive international espionage.