Ultras are notoriously difficult to define. They are the most hardcore and extremist of football fans, but while many groups have become criminal gangs, morphing into semi-secretive, paramilitary organisations that accumulate great power and wealth, others are idealistic crusaders against injustice and tyranny. Plenty of ultras are neo-fascist, but there are many far-left groups, too. The ultra mentality is all about the local – your street, suburb or city – but it’s also a globalised subculture in which fans thousands of miles apart influence each others’ songs, protests, politics and philosophies.

James Montague has spent many years with them (his subtitle – “among the ultras” – seems a conscious nod to Bill Buford’s acclaimed if flawed book on British hooligans, Among the Thugs). The “1312” of the title refers to the alphabet code for ACAB, an ubiquitous acronym which stands for “all cops are bastards”. It’s that which unites the movement: there is, Montague writes, a “mistrust in any type of authority”. There’s a bloody-minded contrariness to the ultras. It is typified by the word dišpet used by its members from Hadjuk Split: “a term of defiance that roughly means to oppose something no matter the consequences … Dišpet means to be anti-everything.”

Montague is a brilliant and daring guide. Travelling to 25 countries to compare ultra movements around the world, he takes the reader to warehouses, forests, terraces and underpasses. In Albania, a nationalist called Ismail drives with his knees as he loads, cocks and fires a gun. In Indonesia the author is chased by rival ultras armed with machetes. He smokes weed with a fascist mobster in Rome. It’s frequently pretty dangerous: “I had been warned that if we ever got into trouble that I should not, under any circumstances, fall over. ‘If you fall, you’re dead.’”

It’s an immersive account, partly because it is clearly a world Montague enjoys. He hints that he was something of a scallywag in his youth: “As a teenager I must have been arrested a dozen times. Getting caught was almost as big a rush as getting away.” Researching and writing the book was a way to recapture that teenage buzz. After one scrape – and there are many here – he says: “My heart was beating fast and I felt something approaching elation after my escape. I was fifteen again. I turned on the light in the bathroom and stared at myself in the mirror. I was smiling.”

But the first-person is never overdone or naff: there’s enough of it to capture the fix and the fear, but it mainly serves to explain the existential attraction of the ultra life: the absorption of the self into something far greater, into the group, into a tribe or a brotherhood. It’s “a sound”, he writes poetically, “that you could lose yourself into”. In some ways it’s nothing to do with football but all about danger and adrenaline, about vulnerability and protection, about piggy-backing football to enjoy an emotional rollercoaster. Every ultra, Montague writes, has the same “origins story, the same recollection. They looked at the pitch, but were drawn to the danger and the noise of the crowds behind the goals. They didn’t just want to watch football. They wanted to feel it too.”

Montague never shies away from the darkest aspects of this subculture. At Boca Juniors in Argentina the Doce crew allegedly pull in $3m a month. Even if other sources have suggested a lower figure of $400,000 a month, it’s still big money, especially since many come from the slums. The stakes are so high that many ultras have, over the decades, been murdered. Much of that money comes not only from dealing tickets and drugs, but from being hired as foot soldiers to do the dirty work of politicians, unions and narcos, who need boots on the ground. As one ultra in Brazil says: “The politicians, judges, the chief police officers – on one side they reject me. But on the other side, they want to talk to you, to get closer, to arrange something with you.” Very often the ultras are like mercenaries, hired thugs who can fight, intimidate and cajole: “It’s organised like an army,” says a Serbian ultra. “In a very short period of time they can organise ten thousand men here in Belgrade.”

The drunk brawls of British hooligans have been replaced by arranged fights in forests between sober martial arts obsessives

The disorganised, drunk brawls of British hooligans have now been replaced by arranged fights in forests between sober, highly trained martial arts obsessives. So it’s not, perhaps, surprising that – for all their avowed independence from power – ultras have sometimes been pawns in geopolitical scraps. More than 60 per cent of Azov (far-right) fighters who helped liberate Ukrainian Mariupol from Russia came from the ultra scene. The Russian oligarch, Ivan Savvidis (who owns PAOK in Greece) was alleged to have funded Macedonia ultras to campaign against the renaming of the country, which was seen as a prelude to Nato access. The ultras are the hardest edge of football’s soft power.

But just as one is becoming jaundiced about this subculture, Montague offers examples of ultras going toe-to-toe with autocratic states. They were instrumental in protests in Tahrir and Maidan squares in Egypt and Ukraine respectively. They have often been on the frontline of civil disobedience in Turkey, Brazil, Sweden and Germany. They “despair at the commercialisation of the modern world” and often fight, literally, against corruption. Without organised fans, it’s likely that German’s revered “50+1” rule (meaning that fans have a majority say in the running of their clubs) would have been abolished. It’s almost as if only ultras can endow football with the metaphysical profundity to which it aspires. Montague describes one silent stadium protest as “sucking the importance and the significance out of what was happening on the pitch. The 22 footballers seemed small and irrelevant compared to the weight of self-discipline and control that was bearing down from four sides of the arena.”

1312 is such a global tour that the reader occasionally risks jet-lag. The countries, clubs and characters come so thick and fast that it can be, even for an expert, bewildering. But that, perhaps, is the point: the reader is left as woozy and punch-drunk as if they had been on tour themselves, with “the light from skyscrapers smearing past on either side”.

• 1312: Among the Ultras – A Journey with the World’s Most Extreme Fans is published by Ebury. To buy a copy go to guardianbookshop.com or call 020-3176 3837. Tobias Jones’s Ultra: The Underworld of Italian Football is published by Head of Zeus.