Will the young Britons fighting in Syria be allowed to return home and resume their lives?

Writers George Orwell, Laurie Lee and John Cornford - along with 2,300 other British volunteers - took up arms in the Spanish Civil War. But Tony Blair's Terrorism Act 2006 has since made it an offence to take part in military action abroad with a "political, ideological, religious or racial motive"

Joining forces: young British men feature in an Isis video in which they urge Islamists in the West to join them in Iraq and Syria

Whenever the BBC needs a standby to fill a gap in its early-evening schedules, it turns to Dad's Army. David Croft and Jimmy Perry's much-loved Second World War sitcom has over the decades tickled audiences a lot younger than those who saw the nine series on their first outings between 1968 and 1977.

The Home Guard volunteers of Walmington-on-Sea have become national treasures, with the show's quirky comedy of character deepened by our knowledge that the real-life equivalents of these bumbling oldies did once stand in the front line of defence against Fascism.

The Home Guard, or Local Defence Volunteers, given official status on 17 May 1940 and stood down on 31 December 1945, had a remarkable birth. As fears of a German invasion grew in 1940, the force's architect had to fight his own campaign against the scorn and suspicion of military top brass and cautious politicians. But his idea for decentralised self-defence militias caught on fast. By July 1940, it had attracted 1.5 million volunteers. Not only did the Home Guard stiffen morale at a time when Britain had no European allies against Hitler; its members took an active part in conflict by manning anti-aircraft batteries and downing many Luftwaffe planes.

Tom Wintringham, the strategist who had agitated for a Home Guard since 1938, outraged the Colonel Blimps with his polemic How to Reform the Army. He kick-started support for "people's militias" when he opened a training school in guerrilla warfare at Osterley. The authorities tried (and failed) to shut down this nest of "Marxist hooligans", but its principles had already taken root. Wintringham never secured a regular army commission. In 1942, he founded the left-of-Labour Common Wealth Party. But what would have happened today to this oddball soldier who inspired our beloved home-front warriors? As a former "foreign fighter" in an overseas conflict, he could have been subject to a sentence of imprisonment for life.

Then a member of the Communist Party, Wintringham had commanded the British Battalion of the International Brigades at the Battle of Jarama in February 1937. At "Suicide Hill", through an extraordinary combination of pluck and luck, the British volunteers played a bloodily decisive role in the early stages of the Spanish Civil War. They were instrumental in holding back Franco's rebel forces in their advance on Madrid and so helped to safeguard the capital for the Republican government. Although Madrid would fall in 1939, Jarama arguably counts as the most significant armed rebuff for international Fascism until the Battle of El Alamein in November 1942. The human cost proved enormous. In Unlikely Warriors, his definitive account of British and Irish fighters in the Spanish Civil War, Richard Baxell calculates that "of the 630 men who had gone into action on 12 February, only 80 were left unscathed when the battle ended".

Heroes? Not, since 2006, according to British law. Some 2,300 British volunteers fought against Franco in Spain; more than 500 were killed. Although history tends to remember the writers and intellectuals – George Orwell and John Cornford; Ralph Fox and Laurie Lee – most were working-class trade unionists in their late twenties, with 200 Welsh miners among them. In 1996, the government of Spain paid the ultimate tribute to their contribution by proposing an offer of citizenship to every surviving member of the International Brigades.

George Orwell wrote a personal account of his experiences and observations during the Spanish Civil War (Rex) (Rex Features)

A decade after that, and just before the grant of citizenship to every veteran entered Spanish law, Tony Blair's third administration passed the Terrorism Act 2006. Section Five, as presently interpreted by the Crown Prosecution Service, makes it an offence to take part in military action abroad with a "political, ideological, religious or racial motive". The legislation appears to forbid all training or action in a foreign combat. If so, its provisions would have criminalised every Briton who fought in Spain. It would have turned Lord Byron, whose commitment to Greek independence led him to arm and lead a raggle-taggle regiment prior to his death at Missolonghi in 1824, into an outlaw. As for the 6,500 veterans of Wellington's armies who went off after Waterloo to fight against Spanish colonial rule in the battles that led to freedom for Colombia, Venezuela and Ecuador, how could the courts have processed such a lawless throng?

The 2006 legislation currently targets UK citizens deemed to have fought with Syrian rebel groups. Estimates of their number vary wildly but a figure of around 400-500 has gained currency. At least eight have died. The fear of radicalisation, with any link to al-Qa'ida-allied units and above all to Isis treated as a communicable virus, has propelled the hard legal line. In January, 16 Britons were arrested after returning from Syria. Further arrests have followed since.

"Potentially it's an offence to go out and get involved in a conflict, however loathsome you think the people on the other side are," affirms Sue Hemming, the head of counter-terrorism at the Crown Prosecution Service (London Evening Standard, 3 February 2014). "Our Government chooses to have legislation which prevents people from joining in whichever conflicts they have views about. We will apply the law robustly." In May, Mashudur Choudhury of Portsmouth duly became the first Briton to be convicted of "engaging in conduct in preparation of terrorist acts" in Syria.

No sane observer will whitewash the motives and methods of the al-Nusra front or the newly rebranded "Islamic State". If, until mid-2014, some foreign recruits could dupe themselves into thinking that Isis stood for a dogmatic but authentic war for faith against Bashar al-Assad's dictatorship, then the surge into northern Iraq which began on 5 June has blown that façade clean away. Everywhere from Mosul to Tikrit and the gates of Baghdad, the forces of the "Islamic State" have massacred Muslims, prisoners and civilians alike. Now they threaten genocide to Christians. Yet this sectarian mass slaughter may make it more vital than ever to clear a path back to normality for the drifters, dreamers, malcontents and bedroom zealots once attracted by the Isis cult. The risk of an indiscriminate criminal stigma might give the doubters and waverers a reason to stick with the fanatics.

Here, the Spanish Civil War precedent may prove far from trivial. Most International Brigaders espoused noble ideals. Yet they hardly served a noble organisation. The Brigades' incorporation into the Comintern and Soviet foreign-policy aims compromised its democratic credentials. Some dissidents – such as George Orwell – detested this assimilation into a Stalinist International. Homage to Catalonia records his disillusioned anger with the takeover. Orwell chose to fight with Poum, the autonomous revolutionary militia treated as an unruly Trotskyist splinter by the Brigades' leadership.

Objectively, British and other volunteers may have served the interests of Soviet policy at a time of political persecution in Moscow and Madrid. Subjectively, save for an atypical handful of ideologues, they took up arms for liberty and in solidarity with a threatened people. Their trajectory – fully documented in memoirs, interviews, films and histories – could still guide official reactions to the young British men who have again journeyed overseas to fight.

The Spanish parallel proves instructive on another level too. The Brigaders hardly came back to a heroes' welcome. Wintringham's frustration, as the Colonel Blimps kept him out of the Army in spite of his proven gifts of leadership, was commonplace. Before and after the outbreak of war in September 1939, recruiters were told to treat evidence of service in Spain as a marker of potential disloyalty.

In 1939, the Communist connection alarmed the authorities as much as an Isis affiliation now. These fears were mostly misplaced. Exposure to the Comintern power-grab in Spain, followed by fury at the Nazi-Soviet Pact of 1939, had loosened the ties of many fighters to the party. Bitter experience turned them against would-be controllers. Could the killing fields of Iraq serve as the same kind of wake-up call for wannabe jihadis as the Moscow purges of the late 1930s? If so, this is no time for the state to block any exit route from barbarism.

Given Britain's plight in 1940, the blanket ban on Spanish veterans could not last. Some won commissions and many more served in the ranks. Bill Alexander, the final commander of the British Battalion, passed out first in his year at Sandhurst in 1942 before becoming a tank captain. Routine suspicion and discrimination did persist. When he learnt about battle-hardened ex-comrades from Spain wasted on backroom tasks, Wintringham scathingly wrote that "I sometimes wonder if the powers that run this country are determined to lose this war in their own way and without interference". But no one sought to exclude the veterans from society as proven "terrorists".

Young people volunteer for foreign combat for a variety of reasons. Heartfelt belief in the justice of a cause fires many, as does solidarity with those of a similar background or outlook. For others, a simple itch for adventure or boredom with life at home will supply the push. From Wellington's grizzled veterans in the Andes through to the last-ditch defenders at Jarama, British history gives us ample opportunities to understand the urge to go abroad to fight.

Laurie Lee was among the intellectuals who fought in the Spanish Civil War (Getty Images)

Yet today's security-led prism and its "radicalisation" model, with the automatic penalties in place for any returnee, appears blind to every nuance. One British volunteer in Syria tweeted a poster that read "Keep Calm, Support Isis": a spoof of the already much-parodied Second World War campaign to beef up morale. What are the chances that the kid who wrote that poster had watched Dad's Army? Pretty high. If so, he will be many things apart from a bloodthirsty future avenger dedicated to importing holy mayhem on to British streets.

The long-term significance of an overseas adventure for anyone may not be apparent to them, or to others, at the time. But every present or past volunteer in Syria now knows they bear an invisible brand marked "potential murderer", stamped by the agencies of surveillance. In a BBC radio analysis, one British fighter thought it a "slightly surreal" notion to "go back to the UK and start a jihad there". For him, at least: "As to the global jihad, I couldn't tell you if I'm going to be alive tomorrow, let alone future plans."

Just because you hear someone rashly cry "wolf" does not mean that wolves do not exist. Over the past six weeks, Isis in Iraq has shown to the world a savagery almost beyond belief. Its bloody stunts may have emboldened a few would-be butchers. They will have deterred many secret faint-hearts, already in too deep. However, if the near-certainty of UK criminal sanctions closes down your road to reintegration, why not rise to the fanatics' bait? What have you then got to lose?

Not many saints travelled from Britain to the Spanish Civil War – but not many thugs did, either. In both Spain and Syria, idealism, escapism and sheer youthful bravado will have been pretty evenly mixed. After such an episode, you would expect young men to develop in many ways. The Spanish volunteers did. One veteran of the International Brigades became a champion of neo-liberal economics and a mentor to Margaret Thatcher: Sir Alfred Sherman. Another would become Britain's most prominent mainstream trade-union leader: Jack Jones. A third quit all politics to flourish as a character actor: James Robertson Justice. It is hard to imagine a better way to kill off such varied careers than by marking every foolhardy youth, whatever their motives, with a lifelong criminal brand.

"The mountains look on Marathon," rhymed Lord Byron before he took up arms, "And Marathon looks on the sea;/ And musing there an hour alone,/ I dream'd that Greece might still be free;/ For standing on the Persians' grave,/ I could not deem myself a slave." Not a slave, My Lord – but, under Section Five, most probably a terrorist. Just like the "foreign fighter" who dreamt up Dad's Army.

A version of this article appears in 'Critical Muslim 11: Syria', edited by Ziauddin Sardar and Robin Yassin-Kassab, published by the Muslim Institute and Hurst Publishers (criticalmuslim.com)