During the Siege of Madrid in the Spanish Civil War, Nationalist general Emilio Mola told a journalist in 1936 that as his four columns of troops approached Madrid, a "fifth column" (Spanish: Quinta columna) of supporters inside the city would support him and undermine the Republican government from within.:126–127
The term was then widely used in Spain. Ernest Hemingway used it as the title of his only play, which he wrote in Madrid while the city was being bombarded, and published in 1938 in his book The Fifth Column and the First Forty-Nine Stories.
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.
By the late 1930s, as involvement in the war in Europe became more likely, the term "fifth column" was frequently used to warn of potential sedition and disloyalty within the borders of the United States. The fear of betrayal was heightened by the rapid fall of France in 1940, which some blamed on internal weakness and a pro-German "fifth column." A series of photos run in the June 1940 issue of Life magazine warned of "Signs of Nazi Fifth Column Everywhere." In a speech to the House of Commons that same month, Winston Churchill reassured the members that "Parliament has given us the powers to put down Fifth Column activities with a strong hand." 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. During the Nazi invasion of Norway, the head of the Norwegian fascist party, Vidkun Quisling, proclaimed the formation of a new fascist government in control of Norway, with himself as Prime Minister, by the end of the first day of fighting. The word "quisling" soon became a byword for "collaborator" or "traitor."