I've never held a gun before but volunteered to fight in Ukraine's deadly war with Russia

Jack Knight still remembers the day he asked his bosses for time off to go to war. It was February 2022, and Ukraine's president Volodymr Zelensky had just issued an urgent appeal for foreign volunteers to help fend off Russia's invasion.

Knight, an ex-bomb disposal expert with the Royal Engineers, was sure his skills would be useful. So he requested a sabbatical from his warehousing job at online grocery giant Ocado, hoping they'd treat it like a request for a gap year. They didn't.

"They said the company couldn't get involved in politics, even though this was about fighting a dictator," smiles Knight, 32, who was raised on a south London council estate. "So in the end I just shook everyone's hands, handed in my notice, and off I went."

Even if they had kept his job open, Knight's employers would have had good reason to doubt he'd ever come back. Few thought Ukraine's military would last more than weeks against Putin's superpower army, and for ex-British soldiers like Knight, it would be their first experience of fighting as underdogs.

No bomber jets to support them in battle, as in Afghanistan. No helicopters to evacuate them if they were wounded. And, given how many people warned them it would be suicide, nobody but themselves to blame if it all went horribly wrong.

For Knight, however, nothing was going to stop him going. It wasn't just that it was for a noble cause. It was also a one-off chance to see combat - a chance he thought had passed him by, having only enlisted in the Engineers as the war in Afghanistan was winding down. His own great-great grandfather, William Young, won the Victoria Cross in the First World War, and Knight had always dreamed of following in his footsteps.

"A lot of generations of my family have served in the military, and having never served overseas, I felt I hadn't done much," he said.

"I remember the first few missions in Ukraine when we got hit by artillery - it felt totally overwhelming, the shockwaves from the explosions blasting through your body, but I actually enjoyed it. I remember thinking: 'This makes me feel alive."

Knight is one of a dozen volunteers whose adventures in Ukraine feature in my new book: "The Mad and the Brave: the untold story of Ukraine's Foreign Legion".

Most of them I interviewed while covering the war myself as a foreign correspondent - some on the frontlines, some in bunkers and bars, and some in hospital beds while they recovered from injuries.

Their exploits are a snapshot into the biggest voluntary mobilisation of its kind since the Spanish Civil War, when 35,000 volunteers from more than 50 countries joined the International Brigade fighting Franco's Fascists.

According to Ukrainian officials, at least 20,000 foreigners applied to join the Legion, and while exact numbers are kept secret, up to 1,000 or more are Britons.

Not all are battle-hardened Paras or Marines. Among the Legion's ranks are adventure-seekers bored with civilian life, raw novices who've never held a gun before, and ex-criminals seeking redemption. For some, it's a chance to follow in the footsteps of George Orwell, whose time as a Republican volunteer in Spain's Civil War inspired his book Homage to Catalonia. For others, it's akin to a globalised Fight Club, a chance to prove themselves in combat far tougher than the Taliban or Al-Qaeda.

In all, at least 500 foreign volunteers have been killed - among them two of the men I interviewed for the book. Countless others consider themselves extremely lucky to be alive. One is ex-British infantryman Andrew Hill, who was shot and captured during combat in early 2022. He spent four months in Russian captivity, during which he was beaten, electrocuted and threatened with the death penalty as a "mercenary".

He was eventually exchanged along with a dozen other foreign fighters in a deal brokered by the oligarch Roman Abramovic.

It was a combat environment that made Iraq and Afghanistan look like child's play. Few volunteers, though, were quite as out of their depth as Scottish volunteer Douglas Cartner, a Scottish tractor engineer who volunteered despite having no military training whatsoever.

"It felt like the right thing to do, and it was a just cause - not a religious or ethnic war," says Cartner, 29. "I knew I'd be winging it, but my attitude to life has always been: 'I'll figure it out'."

Soldiering, of course, is not a profession where beginners get many chances to learn from their mistakes. But Cartner took the view that millions of Ukrainians civilians were also having to learn how to fight from scratch.

He did his own self-taught military crash course, reading field manuals online, watching YouTube combat videos, and consulting veterans for advice. While the Legion has attracted its fair share of Walter Mitties, dubiously boasting of service in the SAS or Green Berets, Cartner never hid his lack of experience. "People seemed to appreciate me being honest."

Still, he found himself on a steep learning curve on the frontline, where Ukrainian commanders had no time to teach newcomers the ropes.

Manning a machine-gun post on his very first day, he nearly opened fire on five soldiers who crept up towards him through no-man's land.

"I couldn't tell whether they were friendly or enemy forces, but I knew that if they were Russians, a point would come where I'd have to fire at them before they fired at me," he says. "Then one of them waved and it turned out they were friendlies, including another Legionnaire from Britain. I'd nearly wiped them out altogether."

Like Knight, Cartner was soon in the thick of it, taking part in a counter-offensive that recaptured thousands of square miles of north-east Ukraine in late 2022. He also forged close bonds with his fellow volunteers, who came from as far apart as Latvia and Argentina, and whose camaraderie helped him cope with the fear.

"War is as much about protecting your friends as your fighting your enemies - you'll never run away from a fight if it puts your comrades at risk, especially when you know they'll do the same for you."

During lulls in combat, Cartner and Knight would return home for breaks, sometimes struggling to readjust. Unlike regular soldiers after tours of Iraq or Afghanistan, Legionnaires got no homecoming parades or cheering crowds, and no warm words from Britain's leaders. The Foreign Office explicitly advises citizens against fighting in Ukraine - although unlike Britons who fought with Kurdish forces against the Islamic State in Syria, no Legionnaires have been prosecuted.

Volunteers returning from Ukraine also lack the counselling services available to regular Armed Forces veterans, despite often suffering trauma and flashbacks. Back home, Cartner found himself taking cover when RAF jets did training flights overhead. Knight remembers holidaying once at Center Parcs, where the landscaped pine forests reminded him of Ukraine.

"I ended up scanning the trees for good recon points," he laughs. "Then when I was in the bar, I'd be working out the best defensive spot if Russian troops stormed in."

When I first started interviewing Legionnaires for this book back in 2022, I expected most would do one tour of duty, then feel they'd done their bit. But most went back for more - partly to support comrades, and partly because combat proved addictive. Cartner is still out fighting, while Knight is now back in Britain, having won a bravery medal for rescuing wounded comrades from a minefield during a battle. "I like to think that my great-great grandfather who won the VC would have been proud," he says.

Today, he is working as an ordnance technician on large scale building sites, checking areas for old Second World War bombs rather than newly-dropped Russian ones. Having lost several comrades out in Ukraine, he feels grateful just to be alive - and grateful too, for no longer having to sleep in a foxhole for months on end.

Long-term, though, he is considering returning to Ukraine - not to fight, but to live. All Legionnaires are offered citizenship, and while Ukraine may be war-ravaged and near-bankrupt, it offers certain things that Knight no longer finds much of in Britain.

For a start, homes are affordable, allowing him a foot on the property ladder that is now-near impossible for his generation in Britain. More importantly, he likes their culture, where self-reliance is prized, and where old-school warrior values are still celebrated.

"They still celebrate and respect their military, and they're proud of their country," he said. "They don't have the crime problems we have in England now, and people have a pride in their surroundings; you'll see old women sweeping the streets and weeding the pavements, whereas here we just expect the council to do things for us.

"I love Britain, and I'd still be willing to fight for it, but there's a lot we could learn from Ukraine."

• The Mad and The Brave: The Untold Story of Ukraine's Foreign Legion, by Colin Freeman (HarperCollins, £22) is out now