A fifth column (or internal enemy) is a group of people who undermine a larger group or nation from within, usually in favor of an enemy group or another nation. The activities of a fifth column can be overt or clandestine. Forces gathered in secret can mobilize openly to assist an external attack. The term is also applied to organized actions by military personnel. Clandestine fifth column activities can involve acts of sabotage, disinformation, espionage or terrorism executed within defense lines by secret sympathizers with an external force.

The term "fifth column" originated in Spain (originally quinta columna) during the early phase of the Spanish Civil War. It gained popularity in the Republican media in early October 1936 and immediately started to spread abroad.

The exact origins of the term are not clear. Its first known appearance is in a secret telegram dated 30 September 1936, that was sent to Berlin by the German chargé d'affaires in Alicante, Hans Hermann Völckers [de]. In the telegram, he referred to an unidentified "supposed statement by Franco" that "is being circulated" (apparently in the Republican zone or in the Republican-held Levantine zone). This "supposed statement" held that Franco had claimed that there were four Nationalist columns approaching Madrid, and a fifth column waiting to attack from the inside. The telegram was part of the secret German diplomatic correspondence and was discovered long after the civil war.

The first identified public use of the term is in the 3 October 1936 issue of the Madrid Communist daily Mundo Obrero. In a front-page article, the party propagandist Dolores Ibárruri referred to a statement very similar (or identical) to the one that Völckers had referred to in his telegram, but attributed it to General Emilio Mola rather than to Franco. On the same day, the PCE activist Domingo Girón made a similar claim during a public rally. During the next few days, various Republican papers repeated the story, but with differing detail; some attributed the phrase to General Queipo de Llano, while later some Soviet propagandists would claim it was coined by José Enrique Varela. By mid-October, the media was already warning of the "famous fifth column".

Historians have never identified the original statement referred to by Völckers, Ibárruri, Girón, de Jong, and others. The transcripts of Francisco Franco's, Gonzalo Queipo de Llano's, and Emilio Mola's radio addresses have been published, but they do not contain the term, and no other original statement containing this phrase has ever surfaced. Australian journalist Noel Monks, who took part in Mola's press conference on 28 October 1936, claimed that Mola referred to quinta columna on that day, but by that time the term had already been in use in the Republican press for more than three weeks.

Historiographic works offer differing perspectives on authorship of the term. Many scholars have no doubt about Mola's role and refer to "fifth column" as "a term coined in 1936 by General Emilio Mola", though they acknowledge that his exact statement cannot be verified. In some sources, Mola is named as a person who had used the term during an impromptu press interview, and different—though detailed—versions of the exchange are offered. Probably the most popular version describes the theory of Mola's authorship with a grade of doubt, either noting that it is presumed but has never been proven, or that the phrase "is attributed" to Mola, who "apparently claimed" so, or else noting that "la famosa quinta columna a la que parece que se había referido el general Mola" (the famous fifth column that General Mola seems to have referred to). Some authors consider it possible if not likely that the term has been invented by the Communist propaganda with the purpose of either raising morale or providing justification for terror and repression; initially it might have been part of the whispering campaign, but was later openly floated by Communist propagandists. There are also other theories afloat.

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.

The notion of a fifth column caught the popular imagination of the public across Europe at the start of the Second World War, especially when people were faced with the rapid occupation of Norway and Denmark by the Nazis, and then the collapse of Belgium, France and the Netherlands. 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. Reports of treachery were common, and when the French premier Paul Reynaud announced that "the bridges over the Meuse had been betrayed", a BBC employee wrote, "I have no doubt that German thoroughness has succeeded in planting a fifth column at vulnerable points." On 23 May 1940, the month after Germany invaded France, the British government under newly appointed prime minister Winston Churchill banned the British Union of Fascists under the Treachery Act 1940. 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 on 4 June, Churchill reassured MPs that "Parliament has given us the powers to put down Fifth Column activities with a strong hand." In July 1940, Time magazine referred to talk of a fifth column as a "national phenomenon".