For Chelsea, this was not just a defeat, but a lesson. Perhaps, if they are smart, it was a lesson they will not have to learn twice. The memories will surely scar too deep. A terrorised Gary Cahill dawdling through the game as if in a chance. N'Golo Kante drifting around at just a fraction of his normal intensity. The aging Branislav Ivanovic, so imperious just a couple of seasons ago, now impotently chasing after red shadows like a third, even older Klitschko brother. At least the Chelsea fans were still in fine voice, singing into the gaping void that may as well have been their defence.
For Arsenal, this was not just a victory, but a manifesto. Alexis Sanchez was virtually unplayable up front. Their third goal was a thing of crystalline beauty. Then, having thoroughly dismantled Chelsea in the first 45 minutes, they defended with equal resolve in the second 45 to secure a win that was both thoroughly deserved and thoroughly surprising. For a team who know what they want but frequently seem to have little idea how to get it, this was a pretty convincing blueprint.
Most of the time, Antonio Conte is a one-man hive of activity on the touchline. He shouts, he gesticulates, he minutely adjusts, like a harried wedding photographer trying to get everyone to squeeze together just a little more. But now, with the game gone, he stood motionless. He had been outmanaged by Arsene Wenger, and Chelsea had been outplayed by a team determined to prove that this season - this time - things would be different.
Even Wenger's team selection was a statement of intent: no place up front for either Olivier Giroud or Lucas Perez, and Sanchez at the vanguard of a rotating front four that proved so successful at Hull last weekend. It was, quite nakedly, an attempt to capitalise on Chelsea's biggest current weakness, and one that again was to cost them dearly: the lack of mobility in their defence.
One of Conte's buzz-phrases is the frequent entreaty to his back four to "feel danger". As he stormed down the tunnel at half-time, his side 3-0 down, it was fair to surmise the message was not getting through. How the Emirates bayed and roared: the primal noise of a crowd that senses nights like these do not come around all that often.
Rarely in recent times - even in the last yelps of the second Mourinho era - have a Chelsea side been so brittle, so leaden of foot, so bereft of simple tactical intelligence. It is easy to blame Conte for this, but with John Terry and Kurt Zouma injured, who else was he to call on? At the very least, Chelsea are reaping the consequences of a poor recruitment strategy and the decision to appoint a coach who would only be available for half the summer.
Throw in the snowball effect of individual errors and low confidence, and you get a first half like the one we saw here. Cahill's evening peaked around 10 minutes in: the point at which he prevaricated over a simple back pass and was robbed by Sanchez, haring out of his blind spot, bearing down on goal and dinking the ball over Thibaut Courtois. It was a classic alleyway mugging.
The first goal created the second created the third. Would Cahill have been standing off Alex Iwobi 25 yards from his goal had he not made such an egregious error just minutes earlier? But Cahill's bashfulness caused him to drop back a couple of yards, which was all the time Iwobi needed to turn and find Hector Bellerin on the right. Bellerin crossed first time; Theo Walcott tapped the ball into a yawning net.
If there was a touch of farce to the first goal and a touch of lassitude to the second, there was a touch of class to the third. A simple ball out of Arsenal's defence was the catalyst for a surgical counter-attack. Mesut Ozil evaded the challenge of Kante as if dodging an ex-girlfriend he had just spotted in the frozen food aisle at Sainsbury's. He slipped the ball to Sanchez, and as Sanchez chipped the ball back across goal, Cahill was stranded in no man's land, resolutely not feeling the danger. Ozil's volleyed finish was scuffed into the ground, but bounced over the dive of Courtois.
What could Conte do? Not very much, in the grand scheme of things. It was perhaps Chelsea's fortune that Arsenal's intensity dropped off just a couple of notches in the second half, safe in the knowledge that the game had been won, and mindful of their Champions League engagement against Basel in midweek. But Conte's switch to a back five, just 10 minutes into the second half, also had an effect. Marcos Alonso replaced the largely ineffective Cesc Fabregas - and how the Arsenal fans loved that - and Chelsea settled into their less ambitious but more achievable target of avoiding humiliation.
They might even have grabbed a late goal or two to spice up the game, but here again Arsenal were just too sharp for them. Bellerin made a superb last-ditch tackle to deny a flying Pedro. Late on, Michy Batshuayi broke clear of the last man, but found his shot blocked by the indomitable Petr Cech.
And so 3-0 it finished: not the drubbing that looked feasible at one stage of the game, but severe enough, and a defeat Chelsea will not forget in a hurry. For a club not renowned for its patience, the rebuild must start now. How many of this back four will take the field when Chelsea visit the Emirates again next season? Not many, I would wager.