What is the Domino Theory?

The domino theory refers to any chain of events set off by a single event.

The Domino Theory is simple. One country falls to a foreign influence, in turn the country next to it falls, and then that one causes another country to fall etc. until all the countries in the area have fallen.

The example contributed above is the classic given for the Domino Theory. If you're wondering why it was related to the Cold War, it was because the Soviet Union used this technique to get all the countries in Europe to be communist, and they were successful because no one would decline them because they were a world super power and they knew they would be attacked if they did decline.