The Cold War began in 1945 after World War 2 between the United States and its allies, and the Soviet Union and its allies. This led to the formation of opposing military alliances like NATO and the Warsaw Pact and tensions including arms races and proxy wars. It also resulted in Soviet espionage efforts in the West through spies like the Cambridge Five who betrayed secrets to Moscow. The Cold War lasted over 45 years until the collapse of the Soviet Union in the early 1990s.