It has taken five seasons of heartbreak, near misses and harsh lessons learned, but Manchester City have now claimed their biggest Champions League scalp by overcoming Barcelona on a pulsating night at the Etihad Stadium.
Hammered and humiliated in the Nou Camp two weeks ago, Pep Guardiola's team made amends by fighting back from a goal down to claim their first competitive victory over the five-time European champions.
But while the result will go down as another significant milestone in the club's Champions League story, the manner of the victory - how Guardiola's players beat Barcelona at their own game - is surely the most important factor from a result which leaves City well-placed to qualify from Group C.
So many City players were outstanding - Kevin De Bruyne, Sergio Aguero and David Silva the pick of the lot - and Guardiola must now use this as a yardstick for his men to match.
Anything but a defeat in Germany in three weeks' time will ensure that City remain confident of progress, with Borussia Monchengladbach faced with the daunting prospect of a decider in the Nou Camp next month.
The absence of first-choice goalkeeper Claudio Bravo through suspension following his red card in the Nou Camp was perhaps not the blow it may have been due to the Chilean's eccentricities, especially with City facing a Barca team weakened by the loss of Andres Iniesta, Jordi Alba, Gerard Pique and the suspended Jeremy Mathieu.
But to end a run of five successive defeats against Barcelona, City needed their A-game.
They started well enough, with Raheem Sterling twice drawing fouls from rash left-back Lucas Digne.
After 11 minutes, Sterling was booked for diving when he had clearly been tripped in the penalty area by Samuel Umtiti. City were incensed, but it affected their concentration and Barca took advantage 10 minutes later.
That City conceded within 10 seconds of Aguero having a shot blocked at the other end was testament to Barca's counter-attacking.
With the ball dropping to Lionel Messi on the edge of the Barca penalty area, the Argentine sprayed a pass to Neymar before racing towards the City 18-yard box.
Messi burst through the middle while Neymar ploughed down the left. And when he received a return ball, Messi was allowed to run through a gaping hole in the centre of the City defence before beating Willy Caballero from 10 yards.
It was too easy and Barca continued to pick City off following defensive mistakes until Ilkay Gundogan hauled the hosts level on 37 minutes.
This time, a poor cross-field ball by Sergi Roberto was cut out by Aguero, who squared to Sterling. He then crossed to the far post for Gundogan to score.
City's equaliser rattled Barcelona, and Guardiola's players sensed and exploited the anxiety in the minds of their opponents early in the second half, quickly taking the lead through De Bruyne's free-kick.
Rarely have Barcelona been forced to take such a prolonged dose of their own medicine, but City went close again when Nicolas Otamendi slid in at the far post, only to miss De Bruyne's cross by millimetres.
Aguero, who was outstanding just two weeks after being dropped for the game in the Nou Camp, then headed wide from close range after leaping highest to connect with Silva's free-kick.
But would City pay the price for so many misses?
When John Stones inadvertently released Luis Suarez on 65 minutes, an equaliser seemed inevitable, but the former Liverpool forward squared for Roberto, whose shot from 15 yards struck the crossbar.
Barcelona could not find a way back into the game and Messi, so influential in the first half, had drifted out of the contest.
But at 2-1, City remained vulnerable to a Barcelona counter until Germany midfielder Gundogan scored again on 74 minutes to make it 3-1.
Aguero's failure to convert a Jesus Navas cross from three yards simply gifted Gundogan a tap-in from close range to seal a famous victory.
That trip to Germany in three weeks' time now does not seem quite so pivotal.
Man City: Caballero, Zabaleta, Otamendi, Stones, Kolarov, Sterling (Navas, 71 mins), De Bruyne (Nolito, 89 mins), Fernandinho (Fernando, 60 mins), Gundogan, Silva, Aguero. Unused subs: Gunn, Kompany, Clichy, Iheanacho.
Barcelona: ter Stegen, Sergi Roberto, Mascherano, Umtiti, Digne, Rakitic (Turan, 61 mins), Busquets, Andre Gomes (Rafinha, 77 mins), Messi, Luis Suarez, Neymar. Unused subs: Cillessen, Denis Suarez, Alcacer, Nili, Marlon.
Referee: Viktor Kassai (Hungary)
Man of the match: Ilkay Gundogan
Match rating: 8/10
There was not a flicker of emotion from Pep Guardiola when the decisive goal which can allow Manchester City to revise their perspective on European football flew in, despatched from a free kick Kevin de Bruyne dipped over the Barcelona wall.