Manchester United must endure a result that will figure in the lore of this fixture for generations. The red card in the 46th minute for Jonny Evans, after he had pulled back Mario Balotelli, was damaging, but Manchester City already held a 1-0 lead by then. While the hosts went on to trim a growing deficit to 3-1 with an impeccably directed drive from Darren Fletcher, that simply galvanised City. They struck three times more in the closing minutes, two of the goals coming from the substitute Edin Dzeko, with a David Silva strike to separate them.
It is almost banal after so thunderous an outcome to turn to the Premier League table, but there must be calm satisfaction in a result that has City emerging with a five-point lead over United. For the football world at large, it is more significant still that City have 33 goals from nine league games. A club of such means does not usually inspire fondness from neutrals, but only a curmudgeon could fail to appreciate the accomplishment of City.
There were several tormentors and Balotelli could be depicted as the originator of this rout since his goals established a 2-0 lead that was ominous. In broader terms, all the same, it was Silva in midfield who did most to bring the destruction that inflicted Sir Alex Ferguson's heaviest defeat in 25 years at Old Trafford. It is impossible to settle on just one culprit, but Evans had looked out of his depth from the start and even when the teamsheet was handed round there was surprise that he had been preferred to Phil Jones.
Ferguson might have yearned for a match-fit Nemanja Vidic, but he is barely over a calf strain and was sent off last week in the Champions League match with Otelul Galati. The United manager was too sensible to make excuses. His observation that Mancini's men had great scope because his own team kept going on the attack was Ferguson's tacit statement that it would have been preferable to settle for losing by a commonplace margin.
It is not a stance typically adopted by the manager and that, in its own fashion, demonstrates just how wary he has grown of a project at Eastlands that has no apparent limit in means or expectation. If Mancini has any misgivings at all they will lie in the realisation that meetings with his side will become ever more important and stimulating for other clubs. The manager will not be fooled either by this outcome, even if it is instantly iconic for fans.
Just last week, City toiled at home to get the better of Villarreal in the Champions League, with the joy erupting from a stoppage-time winner. So far as Mancini is concerned, the beauty of this occasion at Old Trafford was that it would have been beneath United's dignity to take a conservative approach. Even so, neither Ferguson nor the City manager could have anticipated the brittleness of United.
There was, one has to concede, a freakishly inept showing by Evans. With the score at 1-0 for City, he might have changed the character of this game but he miskicked when found by Wayne Rooney in the 40th minute. As it was, City struck the ideal balance between prudence and audacity.
Despite the reputation Balotelli has for volatility, he was preferred to Dzeko at centre-forward. Whatever else is said about the Italian he has been reliable of late and the opener here saw him find the net for a fifth consecutive appearance.
Silva picked out James Milner in the 22nd minute and the low ball to the edge of the area from the left was converted meticulously by Balotelli, who shot past the left hand of the goalkeeper David de Gea. The scorer then lifted his top to show a T-shirt with the message "Why always me?"
One might have assumed that the visit by the fire brigade to his home in the early hours of Saturday morning after a fireworks incident was a matter of sheer chance.
Any hazards at Old Trafford were United's alone. The dismissal of Evans capped a troubled day for the centre-half and raised questions over Ferguson's preference for him. Irrespective of the personnel, though, it seemed almost natural that United should go on to produce their most intense attacking, with Ashley Young seeing one effort blocked before he sent the rebound beyond the post.
City, even so, retained their poise. A flowing move saw Silva turn and unleash Milner inside the penalty area on the right after an hour, with Balotelli at the far post to knock the cutback into the net. This was not one of the days when United could flourish in adversity. Mancini's side were unsparing. Milner released Richards on the right and Sergio Agüero converted his pass for a 3-0 lead.
The bout of even greater punishment started in the 90th minute, with Lescott turning a header from Gareth Barry into the path of Dzeko. The scorer then released Silva to finish. And there was a glorious flourish to come when Silva's vision and accuracy freed Dzeko to capitalise once more. United, all the same, had found their minds reeling well before that.
Man of the match David Silva (Manchester City)