Liverpool v Manchester United Europa League: Respect at the heart rivalry
There is a YouTube clip that gives the clearest possible evidence of the enmity and hostility which have bubbled to the surface whenever Liverpool encounter Manchester United.
It is February 1986 and a youthful David Davies, then a touchline reporter before climbing executive director of the FA, breaks into the transmission to reveal the details of an attack on the United team bus as it approached Anfield.
"A brick rebounded off a window at the side of Mark Hughes," Davies reported.
"Then outside the players' entrance at Anfield, as the players got off the bus, someone sprayed a considerable quantity of liquid, possibly ammonia, in their direction.
"Ron Atkinson and his players ran directly on to the pitch, but 22 spectators, many of them children, suffered just as badly if not worse. A 12-year-old was taken to hospital."
One United fan ended up with a dart in his nose on one trip to Anfield in the 1970s. There have been plenty of incidents at Old Trafford, too.
Yet the attack on United's players in 1986 led to a line being drawn by both clubs. The violence had begun to stain the most celebrated fixture in English football, which will be played on a European stage for the first time tonight when Liverpool host their Europa League last 16 first-leg tie.
While the 30 years since have been pockmarked too often with chants about the disasters at Munich and Hillsborough, the bloodletting has mercifully been banished from the stadiums.
Back in 1986, however, with United returning to Anfield on Boxing Day following the attack 10 months earlier, it required the presence of Bob Paisley, three years after retiring as Liverpool manager, to travel to the game on the visitors' coach in order to promote a more harmonious image.
Bryan Robson, the United captain at the time, recalls: "Bob came on to our coach at the team hotel. He travelled with us to make sure there were no problems.
"I think it turned out to be tear gas that was directed at us a few months earlier. It actually missed most of the players, the Liverpool fans got it worse.
"I remember those fans being invited into our dressing room by our medical staff so they could have their eyes washed.
"That was one of those occasions when the hostility boiled over but, thankfully, those days seem to have gone and there is a mutual respect and grudging admiration now, on both sides."
The enmity is now largely confined to the songs of supporters on both sides. But while a febrile atmosphere will be generated by two sets of fans who save their most biting hostility for each other tonight, behind the scenes relations between the two clubs have never been healthier with Jurgen Klopp, who succeeded Ulsterman Brendan Rodgers, in charge of Liverpool and Louis van Gaal at United's helm.
In November 2012, United quietly donated tracksuit tops worn at Anfield to raise funds for the Hillsborough families' campaign for justice, while Liverpool annually offer their respects, publicly and privately, to United on the anniversary of the Munich air crash.
In 1989, Alex Ferguson was one of the first to contact Kenny Dalglish to offer any assistance United could give in the wake of the Hillsborough disaster, with Dalglish writing to his long-time adversary in August of that year to thank Ferguson.
Alan Hansen, who was at Anfield between 1977 and 1991, insists that respect has always been high on the pitch too.
"We would always share a drink with the United lads after a game," Hansen said. "You can't escape the fact that the fans don't like each other, but there is nothing but the greatest respect between the clubs."
It has not always been a smooth relationship, with Ferguson intervening to prevent United's Argentine defender Gabriel Heinze signing for Liverpool in August 2007 after he had been targeted by Rafael Benitez. "Gabriel was told, with no ambiguity, that historically, Manchester United do not sell players to Liverpool, and vice versa," Ferguson claimed in his recent book, Leading.
Ferguson relished stoking the tensions with Liverpool, despite repeatedly speaking of his admiration of the club.
The Scot would use an incident when Ryan Giggs and Lee Sharpe were asked for autographs, only to have them ripped up in their faces outside Anfield, as motivation, while making it clear that his ambition was to "knock Liverpool off their perch".
For former Liverpool defender Jamie Carragher, though, the fear and loathing make no sense. "The intense competition is more geographical and historical, than logical, to me," Carragher wrote in his autobiography. "Perhaps Liverpool and United are too alike and that's the cause of the problems. Those deep-rooted feelings have never been there for me."
With 122 honours between them - United lead with 62 - glory is what the two share most. Both have suffered droughts in recent years, and a tie in the Europa League sums up their current status. But if a victory over the other can ignite a return to the days of dominance, it will feel as important as any of the previous 194 meetings.