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It was Patrick Vieira, the Arsenal captain, who acknowledged the truth of it this week. "Manchester United," he said, "are like us. They're a big club who want to win a trophy every year. For both clubs the FA Cup is really important."
The arrival of Chelsea as a third force has not diluted the rivalry of the two sides that have dominated English football for the past decade; it has intensified it.
Fairytale upsets having become a thing of the past, previous Cardiff FA Cup finals have either been about completing the double or finding solace having missed out on the Premiership. Appropriately, on Saturday, in the last final there (Wembley's contractors permitting), it is about two teams scrapping for consolation. Thanks to Chelsea's domination of the league, the FA Cup has suddenly become so prized that it means more than mere consolation: mutual desperation has breathed new life into the competition.
Arsenal want a trophy as reward for a season in which the quality of their football has, according to Vieira, "deserved" success; United need one to reassure themselves that their empire is not in terminal decline. Only the gleam of silver could distract from successive third-place league finishes.
Admittedly, we have been disappointed before. The United-Liverpool final of nine years ago promised much only to fizzle into mundane attrition, but the antipathy United and Arsenal share has tended to produce explosive football.
These are two teams who genuinely do not like each other. When Philippe Senderos, so impressive recently that Sol Campbell looks set to start on the bench, spoke of his delight at being compared with Martin Keown, thoughts immediately went to the former Arsenal centre-back's taunting of Ruud van Nistelrooy after a missed penalty last season.
As soon as Arsène Wenger said that Thierry Henry would not be fit for the final, Sir Alex Ferguson decided that "Scottish caution" was appropriate and insisted United would prepare to face him. Every utterance the two managers have made this week has seemed a jibe at the other. Even Wenger's assertion that José Antonio Reyes will start seemed loaded with significance.
For it is Reyes who has borne the brunt of the physical approach with which United have sought to unsettle Arsenal in recent meetings. At Villa Park in last season's semi-final, he was clattered by Paul Scholes within seconds, and then at Old Trafford in October, when United ended Arsenal's 49-game unbeaten Premiership run, he was, in Wenger's words, "kicked off the pitch". When the Spaniard spoke this week of having toughened up since then, you half-wondered whether the anti-bullying wristband he wore was designed to protect himself.
United's robust approach has been undeniably effective. With the exception of this season's Community Shield victory over a weakened side, Arsenal have not beaten United in seven meetings stretching back to February 2003, the game that so frustrated Ferguson that he kicked a boot that struck David Beckham on the head.
Yet in picking Reyes, and announcing his selection so early, Wenger seemed to be saying that Arsenal were not scared of United, that they will not be bullied.
"He's [Reyes] a lot stronger physically than he was at the beginning of the season," Wenger said. "He's an instinctive, central player, so he will always be kicked. What is most important is that the players go out with confidence and try to play." That sounded an almost neutral comment, but the barb soon followed: "After that, it is up to the referee."