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general::
A secret subversive group that works against a country or organization from the inside, as in The rightto-life movement has established a fifth column among freedom-of-choice activists. This term was invented by General Emilio Mola during the Spanish Civil War in a radio broadcast on October 16, 1936, in which he said that he had una quinta columna ("a fifth column") of sympathizers for General Franco among the Republicans holding the city of Madrid, and it would join his four columns of troops when they attacked. The term was popularized by Ernest Hemingway and later extended to any traitorous insiders.
American Heritage Idioms