Where does the term ‘fifth column’ come from?

• Read more | The Spanish Civil War: an avoidable tragedy

What was the fifth column?

Asked which would reach the city first, Mola reportedly introduced a ‘fifth column’ already inside Madrid: “Men now in hiding who will rise and support us the moment we march”.

Madrid, which remained (and would not fall until the very last days of the Spanish Civil War in March 1939) within Republican hands, was on tenterhooks. Many who sympathised with the army’s uprising continued to dwell there. Now, Mola’s psychological warfare put them at great risk, and large numbers of civilians with right-wing views were rounded up and detained within weeks of the coup.

When Nationalist troops were just two hundred yards from one of Madrid’s largest prisons, the Cárcel Modelo – and thus in a position to potentially bolster their fighting capacity – Republicans saw Mola’s so-called ‘fifth column’ among the prisoners.

How did the Spanish Republic respond to Mola’s ‘fifth column’?

The paranoia of sabotage and treachery prompted prime minister Francisco Largo Caballero’s government to relocate the Spanish capital to Valencia on the Mediterranean coast.

Meanwhile, a decision was taken on 7 November 1936 to execute prisoners identified as “fascists and dangerous elements”, while less threatening supporters of the army’s coup were relocated 138 miles southeast to Albacete.

Approximately 2,500 prisoners were massacred by the beginning of December – the single worst atrocity to take place in the Republican zone during the Spanish Civil War.

What did the term ‘fifth column’ mean in the Second World War?

With the outbreak of the Second World War in 1939, the notion of a fifth column as an insidious – and typically ‘foreign’ – uprising bent on destroying society from the inside out found universal currency.

Nazi Germany’s invasion of France in May 1940 compelled the US president, Franklin D Roosevelt, to conjure the prospect of a Nazi fifth column within America during one of his famous ‘fireside chats’ on 26 May.

“Today’s threat to our national security is not a matter of military weapons alone. We know of other methods, new methods of attack: The Trojan Horse. The Fifth Column that betrays a nation unprepared for treachery. Spies, saboteurs and traitors are the actors in this new strategy. With all of these we must and will deal vigorously”, he said.

This fear led to suspicion of foreigners, immigrants and minority groups in America, with concerns of infiltration into key policy-making positions.

What does the fifth column mean in modern usage?

During the Cold War, the term found a new iteration through the Red Scare instigated by Senator Joseph McCarthy’s anti-communist witch hunt in America.

In Britain, during the Miners’ Strike of 1984–5, Margaret Thatcher denounced the National Union of Miners (NUM) as “the enemy within”.

• Read more | How should history remember Margaret Thatcher?

In 2014, Thatcher’s handwritten notes for her speech at the Conservative Party’s 1984 conference in Brighton, were declassified.

They revealed that she had planned to extend this charge to HM Loyal Opposition at the time, the Labour Party. But this was abandoned after the IRA detonated a bomb inside the Grand Hotel, where Thatcher and many of her ministers were staying.

Following the terrorist attacks of 11 September 2001, the New York Post deployed the term ‘fifth column’ in an editorial that suggested Islamists hostile to the United States were “at work on American soil”.