This felt like more than a victory for Arsenal, more than three points. This felt like the moment when Arsene Wenger finally tempered his purist principles to devise a balanced game-plan, making his team more compact and defensively resilient.
And it brought spectacular, deserved rewards. Winning at the home of the champions is not what Arsenal usually do.
Only time will tell whether Wenger continues with this mixture of silk and steel, whether a manager seemingly committed to football as an art form will stick with this plan of stationing more bouncers outside the gallery.
It was as if George Graham had snuck in and written part of Wenger's team-talk. Vanquished Manchester City manager Manuel Pellegrini admitted this was a different Arsenal than usual, a team more defensively adroit.
Before kick-off, Graeme Souness had suggested on Sky that Arsenal were "a team of son-in-laws…there are no Arsenal players who you wouldn't want your daughter to bring home…they're a bunch of really nice guys".
Yet the nice guys turned into warriors, doing to City what teams often do to them. It was not only the local roads embedded with grit.
Arsenal had 10 men behind the ball, playing almost 4-1-4-1 at times with the outstanding Francis Coquelin holding superbly, soaking up pressure and looking to hit on the break. Things went to plan, the new plan. Wenger even managed to get his tricky coat zip done up first time.
This was Arsenal's most intelligent and accomplished display of the season, a special moment for their players, their manager and their fans, who have endured much misery on the road.
So often this season Arsenal have travelled to places like Stamford Bridge, the Liberty Stadium, the Britannia and St Mary's and lost, their open style exposed, their leadership qualities questioned.
Not here. Not at the scene of last season's 6-3 humiliation.
Every one of Wenger's players rose to the challenge, standing up to the champions, inflicting on City their first defeat since November 5. Too often over-reliant on Alexis Sanchez this term, Arsenal enjoyed powerful performances all around.
Santi Cazorla, brimming with endless two-footed invention, was Man of the Match, scoring Arsenal's first goal from a contentious penalty, and creating the second for Olivier Giroud with a free-kick.
Embodying the spirit of collective endeavour, Cazorla also contributed so much defensively, frequently tracking back.
Coquelin ran Cazorla close for the individual honours, building a wall in front of the back-four, bringing leadership, organisation and communication, having the statisticians working over-time, recording his 11 clearances, seven headers, six interceptions, three blocks and two successful tackles. Coquelin almost resembled a latter-day Gilberto Silva.
David Ospina did well in goal, dealing with City's occasional shots. Laurent Koscielny excelled at centre-back. Hector Bellerin was never over-run or over-awed at right-back.
This was more than a victory for Cazorla, Coquelin and company. This was also a significant result for Chelsea, who emerge from the weekend five points clear of City, with the pair meeting next in the Premier League at the Bridge.
City were disappointing, partly because Arsenal were so good at squeezing the space around Sergio Aguero and David Silva, but also because Aguero and Vincent Kompany looked slightly off the pace returning from injury. The home team were missing the drive from central midfield of Yaya Toure, who tweeted his support from the African Cup of Nations. The subtlety of the injured Samir Nasri was also missed.
City lacked sufficient creativity to break down Arsenal's banks of defence. From the start, Arsenal were defending deep, closing down Silva, forcing Aguero down cul-de-sacs and even resorting to the cynical such as when Koscielny took a booking for body-checking Fernandinho.
As the initial sparring took place, the fans traded songs about Samir Nasri and the advertising hoardings urged the home supporters to come to Saturday's FA Cup tie against Middlesbrough. Then Arsenal broke out, Nacho Monreal playing a 1-2 with Giroud and continuing into the box. Kompany moved slightly towards him but the contact was minimal, certainly not justifying Monreal's collapse.
City's subsequent complaints were well-founded. There was certainly no debating Cazorla's penalty, swept to Joe Hart's left.
Arsenal continued to frustrate the champions through some obdurate defending.
Cazorla tracked back to block an Aguero cross.
Bellerin cleared as Aguero threatened. Aaron Ramsey tripped Aguero, stopping an attack, taking a booking for the team. Monreal then repelled a Jesus Navas cross.
Arsenal fans were delighted, and doubtless surprised, by this new mood of resilience.
City needed more support for Aguero, more urgency. Pellegrini sent on Stevan Jovetic for James Milner.
City came more to life. Aguero raced in from the right but his shot was blocked. Ospina pushed away a Navas strike. Fernandinho's shot was deflected over.
Arsenal fans fretted, fearing the onslaught, yet their team stood firm.
Space still opened up for Arsenal to pour into.
Kompany, who had been booked in the first half for a foul on Giroud, then escaped a red for clattering Sanchez as the ball continued to Ramsey, who fired over. Coquelin was still the leader, ordering players around, stopping attacks.
Pellegrini made his second change, removing Fernandinho, sending on Frank Lampard to see if they could claw their way back into this game and get back at Chelsea, who had been so imperious at Swansea City on Saturday. Wenger also made a change, sending on Tomas Rosicky for Alex Oxlade-Chamberlain.
Cazorla had briefly looked tired and there had been a belief that he might have been taken off. Yet keeping Cazorla on proved inspired by Wenger as the diminutive Spaniard lifted in a free-kick that Giroud headed past Hart after 67 minutes.
As Arsenal's players celebrated in front of their supporters, as Cazorla did a wonderful little jig, an inquest began at City. Fernando had failed to stick with Giroud but surely a forward as imposing as the Frenchman should have been the responsibility of Kompany or Martin Demichelis.
Navas tried to get City back into the game but his low shot was dealt with brilliantly by Ospina. With 13 minutes left, Navas was withdrawn for Edin Dzeko, giving City a 4-2-4 look with the width provided by Jovetic and the right-sided Silva.
Closing on a famous victory, Wenger removed Sanchez and Ramsey for a full-back in Kieran Gibbs and a defensive midfielder in Mathieu Flamini. Ospina continued to impress, catching Dzeko's shot. The life drained from City.
Towards the end, Arsenal supporters were twirling their red-and-white scarves in the air as City supporters left in numbers, a few shouting "you're going to win f*** all" on their way out.
If Arsenal play like this, defending like a Graham side, then they may find the right, successful long-term blend.
The final whistle confirmed they had overtaken Spurs to go fifth on 39 points, a point behind fourth-placed Manchester United.
Arsenal's players went over to celebrate with their fans. Cazorla walked slowly back, so exhausted that Oxlade-Chamberlain gave him a piggy-back.
For City, Pellegrini needs to lift his players.
Their next Premier League assignment? Chelsea away.