Merriam-Webster Collegiate Dictionary - domino theory

 noun  Etymology: from the fact that if dominoes are stood on end one slightly behind the other, a slight push on the first will topple the others  Date: 1965  1. a theory that if one nation becomes Communist-controlled the neighboring nations will also become Communist-controlled  2. the theory that if one act or event is allowed to take place a series of similar acts or events will follow