EXACTLY 80 YEARS ago, the Spanish Civil War general Emilio Mola coined an expression that has bedevilled democracy ever since. In the autumn of 1936, Mola was advancing on Republican-held Madrid with four columns of Nationalist troops, when he referred to la quinta columna, the “fifth column”, a troop of secret Nationalist sympathisers within the city who would rise up against the Republican Government when the time was ripe.

The idea of the “fifth column” has become a defining feature of modern political history, expanded to embrace anyone working to undermine the State for outside interests. In many instances, the reaction to the perceived threat has far outweighed the danger itself. Fear of enemy infiltration prompted mass internment during the Second World War, and the McCarthyite hunt for American communists in the late 1940s and early 1950s; before Pearl Harbor, American isolationists were accused of forming a pro-Nazi fifth column in the US.

Today Britain is facing its own fifth column conundrum, threatened by a shadowy group of internal enemies, engaged in a terror campaign against Britain. Like every classic fifth column, this one blends in: the enemies are British citizens, both Asian and white, often middle-class and virtually invisible. The terrorist squatting in a cave on the Afghan border is now a familiar enemy; the alleged bomber working out of a pebbledash semi in High Wycombe is far more terrifying.

Tackling the internal threat poses not just a logistical, but a moral, challenge: how to identify the internal enemy without whipping up anti- Muslim hysteria, how to encourage vigilance without stoking vigilantism, how to see your neighbour clearly, despite the knowledge that the killers could live and work next door.

After last month’s terror plot, Inayat Bunglawala, assistant secretary- general of the Muslim Council of Britain, said: “Some people are keen to portray British Muslims as a fifth column, a mass reservoir of terrorists prepared to slaughter their fellow citizens.”

Some, perhaps, but remarkably few. What seems extraordinary, in the year since 7/7 and the five years since 9/11, is just how restrained the public reaction has been. There have been no calls for mass round-ups, no hate campaigns against Muslims. As shown by the recent Times survey of attitudes among Muslims, they do not appear to have become markedly more isolated and embittered, and thus more vulnerable to infection by extremism. The new confidential hotline set up by MI5 has received numerous calls, but with a few exceptions, these have been genuine and often useful, not the racist finger-pointing and score-settling that might have been expected.

The contrast with an earlier fifth column scare could not be more acute, or more telling. In the run-up to the Second World War, Britain was seized by a spy panic of astonishing virulence, inflamed by the press and politicians. Fifth columnists, terrorists and saboteurs were spotted everywhere, and nowhere; so many reports of suspicious doings flooded in to MI5 that the organisation came close to collapse. “There is a well-defined class of people prone to spy mania,” wrote Winston Churchill, who was not immune to the mania himself. “War is the heyday of these worthy folk.”

Many reports were bogus and xenophobic; some were hilarious. One avid amateur spy-catcher reported seeing a man with a “typically Prussian neck”, and Robert Baden-Powell, the original Scout master, insisted you could spot a German spy from the way he walked. The spies were said to be poisoning chocolate, recruiting mental patients in asylums to act as a suicide squad, and sending agents into the countryside disguised as nuns, butcher’s boys and women hitch-hikers. One secret service officer became convinced that spies were communicating by leaving empty cartons of milk and other detritus in public places — a theory that was, in every way, a load of rubbish.

The most grievous outcome of the panic was the internment of 27,000 Germans, Italians and other “enemy aliens”, most of whom were not only innocent, but opposed to Nazism.

Initially, the failure to uncover the plotters merely reinforced the conviction that the fifth columnists must be terrifyingly efficient. With time, however, MI5 came to realise that there was no organised network of enemies within, but rather a few dangerous individuals who tended to be intensely committed, or slightly mad, or both.

One can see those lessons being applied today. The security services have assiduously avoided a general clampdown on Muslim groups. Information from the public has been encouraged, but treated with caution. There have been serious mistakes, such as Forest Gate, but there is clearly a determined effort to avoid over- reaction: official pronouncements on the threat of Islamic extremism have been deliberately nuanced, and carefully measured.

Every fifth column is built on a foundation. Some Nazi spies were operating in Britain in 1939; some American Communists of the 1950s were slavish servants of Soviet interests; and some British Muslims are terrorists. But history clearly shows that when fear of a fifth column gets out of hand, the danger is blown out of proportion, playing into the hands of the insurgents and unfairly targeting entire communities.

There were Nationalists within Madrid in 1936, though these hardly amounted to a “column”. When the Republican defenders learnt of Mola’s claim about his secret army, real and suspected Franco-supporters were dragged into the streets and murdered in their hundreds.

The response to home-grown British terror suggests that the security services are conscious of history, aware that the enemy within can only be fought with care, realism and reliable intelligence, not with propaganda or by feeding public paranoia. This goes some way to explaining why the anti-Muslim backlash has not happened.

Eight decades after General Mola deployed his rhetorical fifth column, we may finally have worked out how to defeat it.

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