The last time Manchester United played at Liverpool in the FA Cup Alan Smith snapped his leg in two places, his fibula jutting out of his sock like a broken cricket stump, and the Kop baited him with ambulance noises as he was loaded on the stretcher. Coins were thrown at Steven Gerrard and the same, plus a half-eaten burger, at Gary Neville. The ambulance taking Smith to hospital was attacked on the streets outside Anfield and an attempt was made to turn it over. It was a violent, malicious atmosphere even before we get into those warped songs taking spiteful pleasure from the Hillsborough and Munich tragedies.

Incessant hatred is a fact of football life. It's out there, it's unshakeable and everyone who attends these matches is obliged to live with it. This is why the letter Sir Alex Ferguson signed off this week asking United supporters to behave is well-meaning but ultimately futile. They can release doves from the centre circle if they wish but it will make no difference. Liverpool versus United is tribal and the lesson of history, whether we like it or not, is that it is not going to change any time soon, particularly now the Luis Suárez affair has added another dash of poison to the narrative.

The sensitivities are so extreme that Kenny Dalglish restricted his interview time on Friday to an informal gathering of Merseyside reporters away from the television cameras. Dalglish and the Liverpool media department have not always emerged with distinction during a difficult few months but there was clear sense in maintaining a low visibility. Forty miles along the M62, Ferguson was asked his first question about Patrice Evra returning to the ground where Suárez had called him "negro" and United's manager cut the interviewer dead. "I'm not getting into that, right?" It felt deliberate and choreographed and, all things considered, a wise strategy from both camps.

Already, there is talk of United supporters turning up at Anfield with pillow cases in some kind of anti-Suárez stunt. It will probably remain as internet threats. Yet some United fans will almost certainly sing: "Ninety-six is not enough." A larger number consider it repellent, but it still continues. Memories linger of the 2009 night in Porto when, down by the banks of the Douro, a group of United fans had left their pub and were counting the seconds down to midnight to mark the 20th anniversary of Hillsborough.

Some Liverpool fans hold out their arms to make aeroplane impressions and resort to spiteful Munich taunts. A pointless argument rages about which mob started it, and which will sink the lowest. "I've been among otherwise reasonable people who, after a drink or two and in the red fog of the moment, have sung these songs," Stuart Maconie writes in Pies and Prejudice, his journey through the north of England. "That tells you a lot about just how crazy some people can get around football and how deep the rifts and old scores go."

Maconie's story looks at the history behind all the hatred and resentment, going back to the Industrial Revolution and the battles over cotton and taxes that led to the creation of the Manchester Ship Canal – "the civil engineering equivalent of Gary Neville running the full length of the pitch to kiss his United badge in front of enraged Liverpool fans". His conclusion is that "rivalry" is not a strong enough word. "It's a vendetta, a blood feud that's Sicilian in intensity."

All of this should not conceal the fact that, for many people, the tribalism and rawness are among the things that make these occasions special. They consider modern-day football to be too sanitised. However, there is a line of decency and, over recent years, it has been trampled across too often with songs about Harold Shipman, Emlyn Hughes, Michael Shields, George Best, Ken Bigley and anyone else who can be used to score a few points.

"It's different to 30, 40 years ago," Ferguson said in an interview in the New York Times this week, and he cited one of his favourite United photographs from the 1970s, showing the players fighting on the pitch with their Leeds counterparts. "They're ripping the strips off each other. In the background, the fans are silent. They're just watching. Nowadays, they're screaming over the fences, screaming at the away section of fans. The behaviour pattern of people is different today."

In one respect, perhaps. Yet the problem with nostalgia is that it tends to remember only what it wants. Ferguson was United's manager when Paul Parker came to Old Trafford as a player with QPR in the late 1980s and was met with chants of "Trigger, trigger, trigger, shoot that nigger" (Parker pretended to take a gun to his own head). The current Old Trafford offers a very different matchday experience.

Likewise, there has been nothing in Ferguson's 25 years to compare with that day in 1985 when the bus carrying Ron Atkinson's team inched through the Anfield gates and came under attack. "We got off the coach and all of a sudden something hit us," Atkinson remembers. "I thought it was fumes off new paint or something, but it was teargas." Clayton Blackmore was so overcome he could not play. Atkinson described going to Liverpool at the time as like being in Vietnam.

Ferguson's unhappiest memories of Anfield are less violent. The time, for example, a Liverpool fan asked a young Ryan Giggs and Lee Sharpe for their autographs then ripped them up in front of their faces. That was the April day in 1992 when United lost 2-0, blowing the league and taking their run without a title to 25 years. "I learned something that day," Ferguson said a few months later. "I learned something I had not come across despite being a manager for 18 years. I won't be as shocked if I hear a repeat of the Liverpool player who shouted from the dressing room: 'Fuck you.'"

But Ferguson, in his more approachable moments, will testify that one of his favourite memories as a young manager in Scotland was his first visit to Anfield in 1977, to watch the European Cup quarter-final against St‑Etienne. "I didn't walk away from the ground after the game, I floated out," he said at the time. "It was as if I had been given an injection of one of those stimulant drugs. Instead, all that had happened was that I had been caught up in the most exciting football atmosphere I have ever experienced." Many years later, he took exception when José Mourinho attacked the Anfield club. Mourinho, he said, should apologise. "He has insulted Liverpool, a club with great history."

This is the mutual respect that saw John Wark, the Liverpool striker, and John Gidman, the United defender, going for a drink together after the 1985 FA Cup semi-final. Gidman, a Liverpudlian, does recall needing a police escort home one night and Steven Gerrard lays his own feelings bare in his autobiography. "During 90 minutes of football, I want United to die." Yet Jamie Carragher has always had the courage to say how much he admires Ferguson – even if he does offer the kind of caveat that may be expected from someone who gives the impression he probably has You'll Never Walk Alone as his ringtone. "If I was in his company, I would tell him first off that Manchester United never knocked Liverpool off their perch, as he put it. That's nonsense. Graeme Souness did that."

Ferguson's letter, to be handed out at the away turnstiles, asks United's 5,300 fans to be "positive, witty and loud". He was at Anfield for the FA Youth Cup tie last March when a flare was set off, six fans were ejected and, without the usual din of a full stadium, the Hillsborough chants were even more excruciating than usual. On Wednesday, when Manchester City played at Anfield, the Kop held up a banner that read: "We're not racist, we only hate Mancs." At the League Cup final in 2003 it was: "Don't bomb Iraq, nuke Manchester." Last season, at Old Trafford, it was: "Paul Webster is a grass." Webster was the United fan who was attacked on Stanley Park before a game in 2009. In court it was said he had feared for his life as he was kicked and punched to the floor and heard shouts of "cut him". Ten Liverpool fans were convicted.

The whole mood at Anfield is now about removing as much sting from the occasion as possible. Dalglish dodged Sky to avoid any sensationalistic bulletins. In Manchester, Ferguson emphasised that he enjoyed going to Anfield and maybe, late on Saturday, the FA will reflect on a successful day. Until that point the best they can probably hope for is that the police and stewards are on top of things – and that, most of all, it does not go to a replay.