"The story that follows is about Minnesota. It evokes the abstract landscape of our childhood - a bleak, windswept tundra, resembling Siberia except for its Ford dealerships and Hardees restaurants. It aims to be both homey and exotic, and pretends to be true." - from the preface to the Fargo Screenplay

Incongruously placed in the corner of the Fargo-Moorhead Visitors Center, past leaflets advertising such local treats as the Rookery Rock Winery and the Buffalo River Pumpkin Patch, is a rusted-yellow Yard Shark wood chipper. Visitors come from all over America to don a bomber hat, grab the kindly provided fake leg - complete with authentic white wool sock - and reenact the most famous body-disposal scene in Hollywood history.

Such is the enduring legacy of the Coen brothers' Fargo, which cemented them as America's foremost re-inventors of the crime drama. In 2006, ten years after it's release and two Oscars later, Fargo was inducted into the US National Film Registry, one of just five films in the registry to have been awarded a place on the first year they were eligible - an honour shared with Goodfellas, Raging Bull, Toy Story and Do the Right Thing. Now, ten years later, Fargo is celebrating its 20th anniversary.

Grand Guignol in small-town America, Fargo balanced a careful mix of affectionate melodrama, uncompromising black comedy, shocking violence and genuine tragedy. With knife-edge suspense, beautiful low-key cinematography and exceptional performances across the board, Fargo excited audiences and critics alike. It's often listed as the best of the Coen Brothers and still remains perhaps the most precise vision of the famously exacting duo. Fargo has such clear control of place, character and theme that the anthologised television adaptation was easily imaginable before it came to be. All these aspects make it a truly great film but, more than that, it's the Coens' unique post-modern revisions of American traditions that make it such a cultural icon.

1996 was bookended by two events that illustrate the context in which Fargo became, and still remains such an iconic American film. In January, one of the worst snow storms to ever hit the US took 150 lives in the eastern states. In the following December, six-year-old JonBenét Ramsey was found beaten and strangled in the basement of her family home after a supposedly botched kidnapping - an unsolved case that still makes occasional headlines to this day. Fargo tells a similar small-town horror story, reaching into a deep well of American culture that has always displayed a fascination with two forces that defy control and understanding: the wilderness and evil.

Latent destruction hiding under the supposedly civilised and tamed has been a preoccupation of American fiction since early frontier novels. A process of evolution in which civilisation met nature at its most savage, America's frontierism shaped both a pragmatic individualism and a shared culture for the numerous immigrant populations that made up the pioneers. Epic novels of struggle and survival like My Antonia by Willa Cather, characterised the American wilderness as a munificent but capricious god. The winters in the novel are "old and sullen", and Cather's narrator remarks that the arrival of such brutal cold was "as if we were being punished for loving the loveliness of summer." Sentiments that the Coens share, but Fargo's agrestic void is a more dark and cynical twist on the American frontier.

The film begins in a whiteout that suggests less a season than a constant state of blizzard. The shape of a car and trailer slowly appears - little different from the stagecoach convoys of Wells Fargo that enabled the westward expansion - an image reversed at the end of the film, with a convoy of police vehicles disappearing in a flurry of snow. From the icy windshield that frustrates William H. Macy's Jerry Lundergaard into apoplectic rage, to the snow that swallows the blood money buried by Steve Buscemi's Carl Showalter, nature is as treacherous as any hired thug. In the end, it's Minnesota's all-encompassing snow and ice that claims most of the ill-gotten gains. Were the wintry landscape to melt and reveal the satchel of money to be gone, you'd hardly be surprised.

It's not just money that the Minnesota landscape consumes. The Yard Shark wood chipper that mulches Steve Bucemi's pitiful Carl is more than just a dark visual gag and tourist trap novelty. Like an antithesis to the symbolic plough in Cather's My Antonia, which romantically symbolised the heroic pioneers' force of will in changing the landscape, the Coen's wood chipper declares that you're just as likely to feed the American wilderness as feed from it. It's a pointed choice that the film may be set in good ol' Minnesota farmland, but it's commodified diner- and fast-food that characters are seen snacking on.

As a connecting thread between the past and present, the musical motif that recurs frequently throughout the film is an adaptation by composer Carter Burwell of Norweigan folk song Den Bortkomne Sauen, meaning The Lost Sheep. This choice doesn't just relate to the Scandinavian heritage prevalent in Minnesota, but also to the Parable of the Lost Sheep in the Bible, which told of the recovery of lost sinners. Fargo is also a moral parable, but one in which sinners aren't recovered. Part of the long history of America's fascination with true crime, from the Salem Witch Trials to Serial, is the profound effect of puritan moral absolutism and demonology. But the lesson the Coens teach isn't in Bible black and white. Here appearances and absolutes are deceptive, especially when it comes to good and evil.

From the now famous pre-credits proem declaring that 'THIS IS A TRUE STORY', Fargo played with the audience's assumptions. The events depicted in this film did not take place in Minnesota in 1987, but during initial publicity, and even interviews later, the Coens would do much to obfuscate the truth, or lack of it, behind Fargo. Their manipulative myth-making - like the muddled "fakelore" of Paul Bunyan, the giant whose statue marks a welcome to Brainerd - allows for their absurdist world-building with all the trappings of real tragedy. They set up a true crime story, and ensuing audience expectations, in order to make the most of their subversions.

The most immediate subversion of the crime genre encountered is the jaunty tones of Fargo's caricatured Minnesotan dialect. The exaggerated singsong accent and quirky speech patterns in Fargo are used for comedic relief, but beyond the joke is pointed comment on a truly American banality of evil. Underneath the 'Minnesota nice' of Jerry Lundegaard's cheery everyman persona is a ruthless sociopath and conman, who destroys his own family to save his own skin by covering up his failing fraud.

Lundegaard is the quintessential modern American liar, a car salesman, just as comfortable with dishonesty about anti-rust sealant at the car-dealership as he is in executing his wife's kidnapping. The difficulty with which the wronged customer at the dealership forces out an accusation that Lundegaard is a "f-f-fucking liar" shows how tricksters like Lundegaard hide here. His monstrous nature is not just obscured by his veneer of friendliness, but protected by the folksy civility and assumptions of decency of those around him.

While Lundegaard represents the deceitful evil in the ordinary, Frances McDormand's Marge is the authentic counter-point. Her okey-dokey charm isn't a facade like his, but clearly comes from a belief that goodness begins at manners well before it reaches the morality of grand crimes. All the betrayal Lundegaard submits his family to is contrasted with the simple and honest family values in the Gunderson home.

Marge isn't some innocent naïf, however. Although she appears contrary to the classic hard-boiled detective figure - with the Coens slyly inviting us to laugh at a cheerful pregnant woman facing criminal bloodshed - Marge is a keen and determined police chief (it's her stay-at-home artist husband who's the delicate flower.) She's the only one smart enough to see through Lundegaard's disguise, pushing him to ditch his nice-guy act and "get snippy", as Marge chides him.

Marge is able to break the spell of Lundegaard's Minnesota nice after a meeting with an old school friend, Mike Yanagita. Marge takes pity on Mike, who recounts the tragic tale of his wife's death from leukaemia. But a phone call with another friend reveals the truth to Marge; Mike was never married, and the story was just an attempt to seduce her. It's her understanding of this deception ("Oh jeez! That's a surprise!") that leads her to question her previous assumptions about Lundegaard, and the half-baked scam unravels.

This challenge to her assumptions doesn't lead to the standard cynical re-evaluation of beliefs, though, as she stumbles in the snow towards a showdown with Peter Stomare's murderous grump, Gaear Grimsrud. Her difficulty grasping the motivations for all the violence is a riposte to the tired cliché of Nietzschean abyss-gazing in so much detective fiction. "And for what?" she asks a stone-faced Geaer, "For a little bit of money. There's more to life than a little money, you know. Don'tcha know that? And here ya are, and it's a beautiful day. Well. I just don't understand it."

That Marge isn't changed by the white abyss of Fargo's meaningless horror is a victory; the real confrontation not being a gun fight in the snow, but a motherly telling-off in her police cruiser. The Coens aren't just confronting genre and gender stereotypes, but also confronting the audience's expectation that there's something to be understood from the murderers in such stories - true or otherwise.

What makes Fargo an iconic film is how it simultaneously evokes and upends the frontier and crime narratives that inspired it. It took noir to the wood chipper, a genre that had become theatrically stark over time despite the morally ambiguous intent of its beginnings. Fargo may be white with snow, but its story is all shades of grey. Crime can be both evil and incompetent, the wilderness may be brutal but you can easily duck out for a fast food restaurant, a snowstorm can swallow you whole and still be a beautiful day, and pregnant women make great manhunters, but they don't have to hunt like men. A lot can happen in the middle of nowhere.