In July 1936, London garment workers Nat Cohen and Sam Masters cycled across France towards the Spanish city of Barcelona in order to watch the People’s Olympiad, a left-wing alternative to Hitler’s upcoming Nazi showcase, the Berlin Olympics.

They never saw any sport, however, since by the time they arrived, a fascist-backed uprising led by Francisco Franco and other generals had degenerated into civil war. Instead, Cohen and Masters joined the militias that emerged to defend Spain’s Republican democracy against Franco and his allies, Hitler and Mussolini.

That decision placed them in the vanguard of Jewish resistance to fascism, and another 5,000 Jews from across the world eventually followed them to fight in the Spanish Civil War —mostly as members of a remarkable volunteer army, the International Brigades.

“The International Brigades became the vehicle through which Jews could offer the first organised armed resistance to European fascism,” one of them, the American historian Albert Prago, observed later.

The International Brigades attracted 35,000 volunteers from 80 of today’s countries. It was organised by Comintern, the Communist International based in Moscow, but welcomed almost any recruit who declared themselves an anti-fascist.

National contingents brought two dozen languages, but most had one thing in common — a high proportion of Jews.

They accounted for 350 of the 2,400 British volunteers — or 15 per cent — and up to a third of the Americans. Overall, at least one in ten volunteers were Jews. Some estimates double that.

All were leftists, and half were communists. Many were already emigrants who had fled political repression or poverty in central and eastern Europe. Their battle was not just to defend democracy in Spain, but to stop Hitler and Mussolini spreading fascism across Europe.

“All Jewish volunteers understand the importance of the mission they have to fulfil as chosen fighters of the Jewish people,” declared the Yiddish language newspaper of the mostly Jewish Botwin company of the International Brigades.

In fact, scholars disagree over how many of these leftist idealists consciously identified as “Jewish fighters”, rather than as atheist internationalists.

Some came from left-leaning groups like the Geserd, or Society to Settle Working Jews on the Land in the USSR, which encouraged emigration to a specially created Jewish settlement zone around Birobidzhan, just 50 miles from China.

But even to those who put politics above religion, culture or identity, it was clear that the call to arms in Spain was being answered more keenly by Jews than others, to the extent that Yiddish was sometimes used as a lingua franca.

Sephardi Jews, meanwhile, often found themselves working as translators, since their Ladino language was so close to Spanish. Many of the Jewish volunteers were acutely aware of fighting in a country that had expelled their ancestors in 1492 — though they saw Franco as heir to that.

“I am fighting against those who establish an inquisition, like that of their ideological ancestors several centuries ago,” American volunteer Chaim Katz wrote home.

Some volunteers struggled to explain to their families why it was so important for them to fight in a foreign land.

A letter I discovered in Amsterdam’s International Institute of Social History archive while writing The International Brigades: Fascism, Freedom and the Spanish Civil War (published by Bloomsbury this week) expresses powerfully just how difficult the decision could be. It was written by 23-year-old Belgian Communist Piet Akkerman (who had changed his name from Israel) to his conservative mother, Bluma, who had remarried and moved to Britain.

Workers and Jews, Akkerman argued, shared a history of oppression. “Have not 99 per cent of the pogroms in the world been organised to distract attention from the misery of the people by provoking hatred towards the Jews, while those who are really responsible, the authors of misery, laugh in secret because instead of attacking their power, people slaughter the Jews?” he wrote.

While a “gulf” divided her faith from his atheism, he added, “there is one trait that I have developed clearly; that Jewish stubbornness when it comes to holding on to an idea.”

That trait was shared by his older brother, Emiel, who also travelled to Spain. Piet ended his letter by begging Bluma not to cry and telling her that “your son tries to be a man who both thinks and acts humanely.”

Unfortunately, the Spanish Civil War gave her many reasons to cry. Emiel died fighting in Madrid in November 1936 and Piet was killed two months later.

Within months their partners — Vera Luftig and Lya Berger — travelled to Spain as volunteer nurses, along with a dozen other women from the left-wing Jewish youth group they all belonged to.

Piet’s letter fits what scholar Jaff Schatz has called “the moral affirmativeness, longing for justice, and universalist ethos shared by the Marxist vision and Jewish tradition”.

The senior officer among the British Jews was Samuel George Nathan, a pipe-smoking East End butcher’s son and homosexual who was admired for both his impeccable uniform and his calm under fire.

Irish volunteer and former IRA man Joe Monks recalled how, as a company of English speakers attacked the town of Lopera, Nathan waved his swagger stick above his head and shouted: “Cheer! Give them something to be afraid of!” Amid the fear and carnage, Monks said that “the steadiness of Nathan’s splendid shoulders displayed for me that serenity which we were all striving to acquire”.

Among those who followed him that day were British writer Ralph Fox and the poet John Cornford, both of whom died.

Another volunteer recalled bumping into Nathan calmly settling down for lunch while a battle raged around him. “I saw under a shady tree a table neatly laid with a clean check table-cloth and an elegant person sitting smoking in an armchair. Shells were falling within not so very many yards of him. Naturally and inevitably it was Nathan,” he said.

In one of the most unlucky deaths of the war, Nathan was killed a few days later, when some Francoist bombers turned back from a mission and randomly released their bombs over his position.

Masters and Cohen were both wounded in the first months of the war, though the former recovered sufficiently to transfer to the International Brigades and then die at the Battle of Brunete.

Cohen, meanwhile, came home with a Spanish bride, Ramona Siles García — herself a famously fierce militiawoman.

Between one and two thousand Jewish volunteers died in Spain, while others ended up as prisoners of war, including those in a group sneered at by a Francoist officer as “those Americans, with their Jews, and Negroes and democracy”.

They eventually lost their war, with Franco declaring victory over the Spanish Republic on April 1, 1936. He ruled as dictator until 1975.

Britain, the United States and other democracies had pursued a policy of non-intervention in Spain — appeasing Hitler and Mussolini while trying to stop their own citizens from volunteering.

An emboldened Hitler invaded Poland exactly five months after Franco’s victory.

Jewish International Brigade veterans often found themselves fighting in resistance movements and partisan armies across occupied Europe.

Vera Luftig, for example, played a key role in the Soviet’s Red Orchestra espionage ring in occupied Europe.

At Auschwitz and Buchenwald, Brigades veterans led the camp resistance groups that fought with their guards in the final days and hours before liberation.

For many years, Jewish history overlooked the brigaders. The Cold War narrative turned them into suspect allies of Soviet communism, while in Israel they were scorned for having fought a foreign war rather than (in the case of 250 volunteers who travelled from Palestine) staying home to fight against the Arab Revolt of 1936 to 1939.

Over time, however, that image has changed. Brigaders were mostly not Zionists, but Israel came to recognise that, in their fight, they had been defending all other Jews. At the same time, some veterans emigrated to Israel, finding greater acceptance there than in, say, communist Poland, which persecuted them after the Six-Day War in 1967.

Final acceptance came from President Chaim Herzog (himself a former major-general) in 1986. “At the time of the Spanish Civil War there were 55 million people alive who would soon die during the Second World War. There were also six million of our brethren still alive in Europe who did not yet realise that a sword was poised over their necks. But there were people who realised just what a fascist victory in Spain would mean,” he said.

“In the name of the people of Israel, the principal victims of the Nazis and Fascists, I hereby pay homage to the honour and glory of all those volunteer fighters who used their bodies as a dam against a wave of evil.”

Those words are inscribed on a monument to them in Barcelona, along with praise for “Jewish heroes” and their “glorious sacrifice” from former International Brigades Chief Commissar Luigi Longo.

The monument was erected in a former quarry, the Fossar de la Pedrera, beside the sprawling, hillside Montjuic cemetery where Franco’s firing squads buried victims’ bodies in lime. It is a rare oasis of quiet in an otherwise noisy city.

The International Brigades: Fascism, Freedom and the Spanish Civil War by Giles Tremlett is published this week by Bloomsbury