We're accustomed to seeing Joe Sacco as he caricatures himself in his comic books: round spectacles that are glazed over, big lips, woolly hat, shoulder bag, usually scouring a chaotic conflict zone in an effort to get behind the headlines. Sacco's latest work, The Great War, is a departure from journalistic bestsellers such as Palestine and Safe Area Gorazde. For one thing, he's not in it – or at least he doesn't think he is, though he acknowledges he could have subconsciously drawn himself in at some stage. It's easy to get confused. There are literally thousands of people in The Great War, along with horses, heavy artillery, barbed wire, trenches and, as the narrative progresses, explosions, stretchers and graves.

The book depicts the first day of the battle of the Somme, as it's never been seen before. As no-one could have seen it. Technically it's not a book at all: The Great War is actually one continuous drawing, a 24ft-long panorama narrating the British forces' experience of 1 July 1916, spatially and chronologically, from orderly morning approach to chaotic battlefield engagement to grim aftermath. There are no boxes of text or speech bubbles, no individuated characters, instead Sacco portrays a mass event in painstaking, monochrome, almost technical detail. It's like a cross between Hergé and the Chapman brothers; the Bayeux Tapestry as a silent movie.

Sacco considers that level of detail to be a responsibility. "Whenever I've drawn a refugee camp in Gaza or a town in Bosnia – it's got to be that specific camp, that town. It can't be something to stand in for it," he says. "World war one is one of those wars that has its buffs, and I want to please the buffs as much as I can, but also for my own sake; I want to feel that I'm doing right by the historical moment."

The work took him eight months, first sketching in pencil, over 24 panels, then inking over, then rubbing out the pencil lines. Before that, he spent a week researching in the photo archives of London's Imperial War Museum, collecting so many images of equipment, uniforms and battlefields, he eventually broke their photocopier. In addition, he consulted historians, field service manuals of the time, even YouTube footage of Howitzers being fired, all the better to render the combat faithfully. "You do get fixated," he says. "You start thinking: 'How does that bridle work? I've got images of this horse ambulance from this side, but it has actually got something different on the other side. What is it?'"

In a way, Sacco has spent his whole life researching the first world war. He's been reading about it since he was nine years old. Growing up in Australia, where Anzac day was a public holiday, the subject fascinated more than any other, he says. "Those phrases like 'no-man's land' or 'over the top' were very evocative to a 10-year-old. And the way those books would depict trench warfare, with rats, and soldiers going over the top and being killed almost immediately, was sort of shocking to me, even as a kid. That stuff sticks in your head."

The facts of that first day of the Somme are still stirring and horrifying. It was the largest army the British empire had ever assembled. The preparatory shelling was so intense it could be heard in London. By the evening, 60,000 British soldiers were killed or wounded, with no significant territorial gains. "I think of the first world war as a dividing line between our period and whatever came before it," says Sacco. "It was the moment when industrialism really showed its teeth."

Drawing The Great War was a form of penance for that boyhood fascination – now tempered by a career's experience in the cost of conflict. "When you're drawing, it makes you experience things at a deeper level. Because you kind of inhabit the whole scene," he says. "You inhabit each person you draw. You have to give some individuality to each figure, no matter how small they are. I'm leading them to war, and in a way I'm killing them. It's a very intimate experience."

Compared with his usual intrepid missions, The Great War was at least relatively comfortable to create. "I'm working at my desk and I'm wearing slippers, not boots with mud on them, and I'm sleeping in my own bed. That's kind of nice." And the eight months went by in a flash, he says. "Footnotes in Gaza took me seven years." He's just finishing a new book on mass burials at Srebrenica, but he's not sure he'll be returning to his trademark in-the-field reportage after that. Not for comfort reasons, he stresses. "Journalism allows you to explain a lot, but it doesn't allow you to explain everything. It certainly doesn't allow you to get into the psychology of what's going on. I'd like to look at the same topics but in different ways … maybe more as an artist. I've got to change gears somehow. This piece is a way of doing that."

The Great War is published by Jonathan Cape. Joe Sacco will be in conversation with The Guardian's Steve Bell at Comic Art Festival in Kendal, Cumbria, on 20 October, comicartfestival.com