A fifth column is a group of people who undermine a larger group, such as a nation or a besieged city, from within. The activities of a fifth column can be overt or clandestine. Forces gathered in secret can mobilize openly to assist an external attack. This term is also extended to organized actions by military personnel. Clandestine fifth column activities can involve acts of sabotage, disinformation, or espionage executed within defense lines by secret sympathizers with an external force.
Some writers, mindful of the origin of the phrase, use it only in reference to military operations rather than the broader and less well defined range of activities that sympathizers might engage in to support an anticipated attack. Madeleine Albright, for example, in a lengthy account of German sympathizers in Czechoslovakia in the first years of World War II, reserves it for their possible response to a German invasion: "Many, perhaps most, of the Sudetens would have provided the enemy with a fifth column".
In the United States at the end of the 1930s, as involvement in the European war seemed ever more likely, those who feared the possibility of betrayal from within used the newly coined term "fifth column" as a shorthand for sedition and disloyalty. The rapid fall of France in 1940 led many to blame a "fifth column" rather than German military superiority. Political factions in France blamed one another for the nation's defeat and military officials blamed the civilian leadership, all helping feed American anxieties. In June 1940, Life magazine ran a series of photos under the heading "Signs of Nazi Fifth Column Everywhere". In July 1940, Time magazine called fifth column talk a "national phenomenon". In August 1940 the New York Times mentioned "the first spasm of fear engendered by the success of fifth columns in less fortunate countries". One report identified participants in Nazi "fifth columns" as "partisans of authoritarian government everywhere", citing Poland, Czechoslovakia, Norway, and the Netherlands.
John Langdon-Davies, a British journalist who covered the Spanish Civil War, popularized the term "fifth column" by publishing an account called The Fifth Column in 1940. The New York Times published three editorial cartoons that used the term on August 11, 1940.
German minority organizations in Czechoslovakia formed the Sudeten German Free Corps, which aided the Third Reich. Some claimed they were "self-defense formations" created in the aftermath of World War I and unrelated to the German invasion two decades later. More often their origins were discounted and they were defined by the role they played in 1938–39: "The same pattern was repeated in Czechoslovakia. Henlein's Free Corps played in that country the part of fifth column". Albright uses the term in this context only to describe what did not happen, in that the German invasion met no Czech resistance, obviating the possibility of anyone playing the role of fifth column in the military sense.
In 1945, a document produced by the U.S. Department of State compared the earlier efforts of Nazi Germany to mobilize the support of sympathizers in foreign nations to the superior efforts of the international communist movement at the end of World War II: "a communist party was in fact a fifth column as much as any [German] Bund group, except that the latter were crude and ineffective in comparison with the Communists".Arthur M. Schlesinger, Jr., wrote in 1949: "the special Soviet advantage—the warhead—lies in the fifth column; and the fifth column is based on the local Communist parties".
Robert A. Heinlein's science fiction novel Sixth Column (1949) describes the work of a "sixth column", a hidden resistance movement fighting an oppressive occupying force of Asians on American soil. The novel included many references to the Spanish events in which the term originated, so as to contrast—in the author's view—the traitorous fifth column with the novel's patriotic sixth.
^Robert A. Heinlein, Sixth Column (Gnome Press, 1949), 36: "this would not be a fifth column of traitors, bent on paralysing a free country; but the antithesis of that, a sixth column of patriots whose privilege it would be to destroy the morale of invaders, make them afraid, unsure of themselves." See sffworld.com: Mark Yon, Review of Heinlein, Sixth Column, accessed July 23, 2012
"The German Fifth Column in Poland" London: Polish Ministry of Info, 1941
"Fifth Column at Work" by Bohumil Bilek, description of German minority in Czechoslovakia, London, Trinity, 1945