The first time Arsenal have beaten either of Man United or Chelsea in two years, and despite Chelsea's grim run of form, it was the home side's most accomplished performance since they went to the San Siro and outpassed another Ancelotti-managed team in 2008.
The key was tempo and pressing. At last, Arsenal worked as hard without the ball as they did in possession. They hounded the Blues high up the pitch and when this happens, there is less onus on Arsenal to score the "perfect goal". Arsenal looked most dangerous when Chelsea had the ball in their own half.
This, and the criticism Song has attracted for his apparently over-adventurous style this season, made the nature of the first goal ironic. It was the result of a short passing move, the type that Chelsea usually repel with ease, but this time the weight of Arsenal's numbers undid them.
I worried in the build-up about Arsenal's tendency to overcommit in attack, leaving gaps behind them, and their attendant inability to attack any other way. This time it paid off as the red shirts swamped Chelsea- everyone but the back four ended up in or at the edge of the box. Song passed to Wilshere and continued into the box. The younster flicked it back his way, and Fabregas took over only to be tripped by Ferreira. Both Van Persie and Song were on hand, and the latter provided the finish with his lesser-used left foot.
Coming close to half-time, the goal was thoroughly deserved. Arsenal were comfortably the better side, without creating many clear opportunities. Chelsea looked deflated after their recent travails, didn't deal well with the pressure Arsenal exerted, and only really looked threatening from Cech's kick-outs.
Lampard looked rusty after his lay off. Drogba was matched by Djourou, who was at last allowed to replace the unconvincing Squillaci. The Ivorian had his moments early on. When Koscielny ventured too far into midfield and gave the ball away, one Chelsea pass exposed Arsenal's often fatal flaw, and Drogba was running at a back-pedalling Djourou. His shot was angled just wide. Then he attacked a cross well but was tracked by the Swiss defender who managed to deflect the header wide.
Michael Essien was a shadow of his usual ubiquitous self, and Florent Malouda seems to have gone off the boil completely after being one of the players of 2010. But despite Chelsea's problems, Arsenal deserve praise for exploiting them and, hopefully, banishing that debilitating inferiority complex.
Arsenal's half time lead seemed to raise as many questions as it answered. The memory of the North London derby debacle was fresh and painful.
Within minutes of the restart, the Emirates exploded into the greatest scenes of euphoria seen there in quite a while. Van Persie was crowded out by blue shirts, but Essien only nicked the ball inadvertently behind Ashley Cole and into the path of the onrushing Walcott. He advanced on Cech and slipped the ball sideways for his captain to slide the ball into the unguarded net.
Less than two minutes later, Fabregas returned the compliment. Again, Walcott's anticipation was crucial. Malouda accepted the ball from Terry but was immediately robbed by the Englishman. Fabregas took the ball on, and dinked it over Terry's challenge. Cesc's pass was weighted so that Walcott needn't take a touch before shooting. He again exhibited his finishing prowess, blasting across Cech and into the bottom corner.
The worry remained that Arsenal could blow any kind of lead, and nerves were set to jangling just four minutes later. Drogba's free-kick was flighted superbly, so that Fabianski was left in no man's land when Ivanovic rose above Koscielny to head in. There was a period of uncertainty- at one point Ivanovic broke a couple of tackles and surged into the Arsenal area, but Drogba dithered and the chance was gone- but it soon became apparent that Chelsea lacked the confidence to put anything together in open play, and Arsenal played the game out with only a couple of scares from set plays, and once when Ramires shot wide.
Nasri and Diaby fluffed chances to finish Chelsea off, but overall one could have little complaint about the way Arsenal played. Deficiencies remain- there was almost comical panic at a stoppage time set piece, the ball eventually rebounding off Fabianski into his own net, only for the "goal" to be disallowed- but the high-energy pressing that Arsenal put into action gave them a much better chance of overcoming those flaws.
Song and Djourou probably merit special mention. Walcott and Fabregas were the match winners despite both suffering nervous starts in differing ways. Nasri was always dangerous, especially when delivering a clever chip at 0-0 that Cech had to tip over. Clichy, though not hugely tested by Kalou, hounded and harrassed. Sagna was typically steady, and Koscielny, despite having a few uncertain moments, looked far more assured in the absence of Squillaci. Wilshere was the perfect mix of graft and craft, his growth continues. Van Persie's movement bothered Chelsea and he is still not at his sharpest.
Still, the result should be viewed as more a challenge than an accomplishment. Arsenal must now match this level of performance for the rest of the season. Wenger picked the right team, they played the right way, and the whole club should be re-energised.