A vast crowd of people are singing raucously, raising large beer tankards skyward and grinning like they have just won the lottery. In this small and sweaty venue, a Swedish heavy metal band are opening their set with a song about the exploits of Field Marshall Rommel’s infamous 7th panzer division in the second world war. They follow it with a number about the horrors of Passchendaele in the first world war.

By the end of the night, we will have had exuberant hymns to Lawrence of Arabia, an all-female Soviet bomber squadron and the military genius King Gustavus Adolphus of Sweden. Everyone in the room, the vast majority of whom are wearing Sabaton shirts, sings along with absolutely everything. Meanwhile, the venue gently rocks from side to side, because we are on a ship in the middle of the Baltic Sea.

Welcome to the 10th annual Sabaton Cruise: The Battleship, where passions for military history and Olympic-standard drinking collide. Sabaton may be an unfamiliar name to non-metalheads, but they won’t be for long. Other than veterans Iron Maiden, they are the biggest heavy metal band in Europe. Their last album, The Great War, reached No 1 in Sweden, Germany and Switzerland and No 11 in the UK – very respectable for a heavy metal band.

Facebook Twitter Pinterest Watch the video for Fields of Verdun

What’s more, the band have made significant inroads into the notoriously Euro-phobic US rock market. In addition to their annual cruise (which has sold out every time), they have also been running their Sabaton Open Air festival in their home city of Falun, in central Sweden, since 2008, and have their own history channel on YouTube, where the stories behind their military history-themed songs are explored at length by academics and historians. Every diehard fan crammed into the ship’s mini-theatre has completely bought into the whole thing. The response, as Sabaton conclude the show with pummelling metal jig Swedish Pagans, is deafening and utterly joyful.

The following morning, swarms of brutally hungover fans stagger chaotically towards the ship’s buffet restaurant for sustenance. Others stumble out on deck into sub-zero temperatures for a fag. Everyone seems to have survived the night without falling overboard. For all heavy metal’s reputation for rowdiness, the Sabaton cruise has a lovely family vibe – Mikael, from Stavanger, in Norway, tells me he fell asleep in one of the restaurants, but was helpfully carried to his cabin by some Germans. With a Sabaton quiz, heavy-metal karaoke and a merchandise stall selling a bewildering number of items for all sizes, genders and tastes, it all feels very inclusive and cosy. The band’s obsession with war stories ensures that we are all learning something, too.

Facebook Twitter Pinterest Fans are united by their love of power metal and the minutiae of military history. Photograph: George Grigoriadis/The Guardian

“Every school system I’ve ever encountered has been very effective at turning something interesting, like history, into something boring,” says Joakim Brodén, the band’s mohawked, musclebound frontman. “Of course you have to learn facts and figures, but that can be so boring. If you zoom in and tell stories, like the tale of Witold Pilecki, a guy who went voluntarily to infiltrate Auschwitz and support the resistance? These are human stories – people from every nation and every culture can relate to them.”

The band love military history so much that when they heard that a museum on the site of the UK’s only first world war battle was in financial trouble – Heugh Battery Museum in Hartlepool – they created a special T-shirt design and raised £4,000.

As educational as their songs are designed to be, Sabaton are aware that their obsession with military events, and the grimmest aspects of war, could be misconstrued as ghoulish, flippant or possibly even politically dubious. There’s no doubting that the band’s motivations are entirely honourable, but Brodén does say they have to remain mindful of the emotional power of these tumultuous tales. “We played the song 40:1 on the battlefield in Wizna, Poland, exactly 70 years after the battle of Wizna happened,” he says. “We had the Swedish ambassador there and 10,000 fans, and we were in the field where it actually happened – you could see the fire, literally, in the fans’ eyes. Those songs are often the most fun but they can be a little scary, too. We’re singing about military history because it’s fascinating. We’re not pushing nationalistic propaganda.”

Brodén and bassist Pär Sundström are the sole remaining members of the original Sabaton lineup, and have nurtured their band over two decades: Brodén writes the music, while Sundström seems to be responsible for everything else. Aside from being Sabaton’s manager, he books their shows and deals with logistics, stage sets and a steady stream of new projects he can’t stop dreaming up. “Nobody tells us when or where to play, how to build a stage, how to do accounting,” he says. “Give me a problem to solve and some tools – and I’ll find something I can build with them. I’ll want to build something bigger than everything else.”

This includes the Sabaton History channel. Sundström’s piercing blue eyes light up as he explains the thinking behind it. Conceived to flesh out the military stories behind the band’s songs, each episode has led to a surge in streams and album sales. “It’s definitely not paying off in terms of direct income, but not everything needs to do that,” he says, adding that they are turning their website in “an encyclopaedia of military history. You can see how each day in history relates to Sabaton. So you can read about it, watch it, listen to it – we’ll get you one way or the other!”

On board the “battleship” (in reality a standard cruise ship, albeit a huge one), there is plenty of evidence that Sabaton’s remorseless pursuit of new ways to attract fans has led to the creation of a genuine community. Up to 40 different countries are represented among the cruise’s 3,000 punters, and Brodén seems thrilled to have brought so many people together. “The Polish fan club started hanging out with the German fan club and so on. I must’ve bumped into 100 people I recognised yesterday.”

Facebook Twitter Pinterest Fans join the party in the middle of the Baltic Sea. Photograph: George Grigoriadis/The Guardian

Several unexpectedly successful tours in the US disproved the notion that, as Sundström claims to have been repeatedly told, “Americans don’t like power metal”. But even within metal circles, the Swedes have seldom been favoured candidates for world domination. Sabaton albums have generally been greeted with an appreciative but noncommittal shrug from the rock and metal press. So-called “power metal” – a brightly melodic and fast-paced strain of old-school metal – has never sought to be cool, and is easily ignored. But while many younger, ostensibly cooler bands have risen and fallen, this defiantly traditional quintet have marched stoically onwards, oblivious to time or trends but dragging a phenomenal number of people along with them.

“It’s always been a steady, upward march. We’ve never had a moment where we’ve lost our footing,” Brodén notes, almost sheepishly. “For me, personally, I had one ‘What the fuck?’ moment in Athens in 2009. We were playing in an underground club, not a huge show, and I think we were playing Ghost Division. Suddenly, everything went into slow motion. I realised, ‘Hmm, I’m in Athens having a good time, playing a show, and I haven’t done anything else for the past two years. I guess I’m a professional musician!’”

Facebook Twitter Pinterest Where to next? A fan checks out the brochures on the Sabaton Cruise. Photograph: George Grigoriadis/The Guardian

“We were never welcomed into the music industry, especially in Sweden,” adds Sundström. “We are the outsiders, the ugly guys that they don’t want at their parties. A lot of people thought they could ignore us and we’d go away, but they were wrong. Now they have a big problem because they have to write about us and we’re selling their magazines, but they missed the boat, so to speak.” He hoots with laughter.

The ship is heading back to Stockholm and Sabaton are bringing the cruise to a close with a second lengthy set full of songs of heroism, bravery, tragedy and horror, with historically pertinent visuals blazing from screens behind them. The band are clearly having as much fun as the fans, despite being (we assume) considerably less inebriated. The last song, To Hell and Back, is a blistering paean to the poetry of US veteran and actor Audie Murphy and goes down so well it nearly sinks the ship. Brodén and Sundström bump fists and hug as the diehards chant their band’s name.

“Our music and our lyrics speak the same emotional language,” says Brodén. “In metal, the spectrum can be anything from hatred and aggression through to happiness, joy and pride. That’s all in this music already – and these military stories have it all, too. We didn’t realise it at the beginning, but mixing those two things up was a fucking slam dunk!”

• Sabaton play Wembley Arena, London, on Saturday 8 February