No pasarán: anti-fascist slogan takes on new significance in Ukraine crisis

The phrase is associated with the Spanish civil war, which Vladimir Putin's invasion of Ukraine has made terrifyingly relevant - especially as volunteer fighters from across the world gather to defend the country from his attack.

The brigadiers chose the right side of history. Both Adolf Hitler and Benito Mussolini sent troops to fight alongside the violent rightwing reactionaries led by Spain's future dictator, General Francisco Franco. Like Putin, they wanted to demolish democracy across Europe.

In Ukraine, the president, Volodymyr Zelenskiy, also wants a volunteer "foreign legion" to join the war. "This is the beginning of a war against Europe, against European structures, against democracy, against basic human rights, against a global order of law, rules and peaceful coexistence," he said. "Anyone who wants to join the defence of Ukraine, Europe and the world can come."

The bad news for Putin is that some are already on their way, abandoning cozy homes and loving families in North and South America, Europe and Asia to fight in a foreign field. Over the past few days, I have been talking to the Canadian paramedic Anthony Walker as he travels through Poland towards Ukraine. The 29-year-old from Toronto is in contact with dozens more people who are also travelling, including military veterans from Canada, the UK and the US.

He has left a "distraught" wife and three children behind and hopes to become an army paramedic. "I guess I'll spend half my time healing people, and the other half shooting at people," he told me as he stocked up on supplies at a Polish medical store.

Like many International Brigadiers, Walker believes he will be protecting his own family back home from the global spread of Putin's fascist-style violence if he is not stopped now.

But Spain holds lessons that volunteers should heed. The first, obviously, is that war is deadly. One in five International Brigadiers died, and a similar number were seriously wounded. They also killed a lot of people, many of them conscripts. Another lesson is that, while you can volunteer to join an army, you cannot always freely leave one. Armies decide for themselves if, or how, that happens. Deserters are punished.

It is dangerous to fall prisoner. International Brigadiers were routinely shot, while those caught in Ukraine today also risk being exhibited as supposed proof that the "wicked west" wants to kill Russians. The Putin regime already claims they are mercenaries who are not covered by the Geneva convention.

Finally, their own countries may not let them fight. When the UK foreign secretary, Liz Truss, encouraged people to volunteer, the prime minister's office suggested it may be illegal.

International Brigadiers found themselves spied on by intelligence services such as the FBI or MI5, in part because they were not all diehard democrats. A significant number were Stalinist communists. The Stasi, East Germany's secret police, would be founded and run by brigade veterans.

Looking at the first pictures of volunteers in the Ukraine, whether doing TikTok dances from the Indian subcontinent or daubing themselves with war paint, it seems obvious the legion will attract all types. Ukraine's military must assert its control.

In war, you can often join people you usually disagree with. That is how the capitalist west and Stalin's Soviet Union ended up on the same side in second world war.

The moral dilemma provoked by war was best put by the French philosopher Simone Weil, when she signed up for Spain aged 27 and shed pacifism. "When I realised that, despite all my efforts, I could not avoid taking part morally in this war, by which I mean that every day, at all hours, desiring the victory of one side over the other, I told myself that Paris was like being in the rearguard and took a train to Barcelona," she wrote.

Those who are injured, mortally or not, may well have last-minute regrets. George Orwell felt "violent resentment at having to leave this world which, when all is said and done, suits me so well" after being shot through the neck in Spain. He survived, and while he hated the thought of dying, he never regretted fighting there.

Another lesson from Spain is that being right does not ensure victory. Franco won the war, ruling as dictator until 1975.

Brigadiers, however, were fighting another war - about moral truth - that they did eventually win. While politicians in western democracies were too terrified of Hitler to send troops or weapons, the brigadiers argued that fascism must eventually be stopped with arms.

When Hitler started the second world war five months after the Spanish war ended, they were proven right. The allies' final victory meant that the wider aim of stopping fascism was also achieved.