It has been a 16-year rivalry, a sporting lifetime of enmity that turned into an unlikely entente cordiale, but the most telling victory of all has occurred off the field. By agreeing to sign Robin van Persie from Arsenal, Sir Alex Ferguson has inflicted one of Arsene Wenger's most humiliating defeats. In its own way, it is as damaging as the 8-2 thrashing at Old Trafford last August.
Van Persie played that day, missing a penalty but scoring a goal in a game that seemed to crystallise the identities of the two clubs: Arsenal had promise, but United performed. Perhaps it made the Dutchman's decision clearer. As his contract ticked down, he eventually announced he would not sign another. But while his parting gift came in the form of an astonishing eight months of sustained success in the penalty box, his 30 goals propelling Arsenal back into the Champions League, his departure will distress them.
They have served as a feeder club for Barcelona and Manchester City before, but the ideology of the purists and the affluence of the super-rich served as an explanation. This is different. This is United. This is proof that Ferguson's intense competitive nature has translated into the transfer market. It is also a unique deal.
He failed in a long pursuit of Patrick Vieira a decade ago. None of the Arsenal greats, of either the George Graham or the Wenger eras, headed for Old Trafford. Meanwhile, the only man to make the opposite journey during the Franco-Scottish war was Mikael Silvestre, and that was something of a hospital pass from Ferguson to Wenger. The Frenchman proved to be perhaps Arsenal's worst defender of recent times. The Dutchman is certainly their best player of late, their finest forward since Thierry Henry.
In terms of money, Van Persie is United's largest signing since the club record buy of Dimitar Berbatov in 2008. Rio Ferdinand, Juan Sebastian Veron and Wayne Rooney cost too. Forwards, as Ferguson illustrated with the catalytic purchase of Eric Cantona, are likelier to determine the destination of trophies. And while Rooney has done that, he, like the still more influential Cristiano Ronaldo, was a teenager, bought for his potential rather than his pedigree. United life was a learning process for both.
By signing the most coveted player in English football, perhaps the closest comparison is the 1993 arrival of Roy Keane. Even then, however, the Irishman came from relegated Nottingham Forest, not a title rival. So Van Persie stands alone.
At 29, he is a rare example of immediacy in United's transfer policy. Ferguson has adopted a futuristic attitude, tying one hand behind his own back with a reluctance to buy players aged 27 or over since the £30.75 million arrival of Berbatov. Now, finally, he has not paid for potential but for a proven, world-class player. Like the Bulgarian before him, Van Persie will have little resale value after three or four years. He is brought in to be prolific, not to make a profit.
He is about the here and now, about wresting the title back from City's grasp and restoring United to the position of pre-eminence that Ferguson prefers. He also has the attributes to be a quintessential United player, the blend of the aesthetic and the arithmetic, the style and the statistics. The classiest volleyer of a ball in England and the possessor of an outstanding left foot, he has an angular, educated excellence. But football is also a numbers game and Van Persie's figures were fantastic last season: 37 goals last season, 30 of them in the Premier League, where he chipped in with nine assists.
Between them, he and Wayne Rooney scored 72 club goals in 2011-12. Do that together in 2012-13 and United could be formidable. They have the promise of a fluid forward partnership, perhaps with the Englishman starting behind the newcomer, but each able to drop off and create. Pity the poor defenders that encounter them, not to mention the able attackers sidelined by them. Danny Welbeck and Javier Hernandez begin to take on the appearance of deluxe substitutes; Berbatov could look still glummer and paler sat in the stands, unless he is slung out of the Old Trafford exit.
After failing in his bid to buy Lucas Moura and Eden Hazard, Ferguson has changed the narrative of his summer - indeed of everyone's. But, as Arsenal had resigned themselves to their captain's departure, it is a deal that says more about United. The Scot will be able to argue that the owners do not hamper his ability to spend. Roberto Mancini is likely to bemoan City's inability to bring in blue-chip players.
That long appeared to be United's lot as they were outbid - or opted not to bid - as City and Chelsea, Real Madrid and Barcelona were the eager buyers. Van Persie's arrival marks a seismic shift in their approach and, before a ball has been kicked, a turning point in the title race. Having announced his decision to leave Arsenal on American Independence Day, United's bid was accepted on Indian Independence Day. From Miami to Mumbai, from New York to New Delhi, both were big news. Essentially, though, it is about the red halves of North London and Manchester and about two veteran managers - one turned Van Persie into the most feared forward in the country and one who then signed him. He is Ferguson's greatest transfer-market coup for a long, long time.