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 player 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.

Guardiola: Man City beat 'the best team in the world'

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 players to match from this point on.

Having cruised to a 4-0 victory against Monchengladbach on match-day one, City appeared to have given themselves the platform on which to build a stress-free qualification campaign.

But the two dropped points against Celtic in Glasgow, when the Scottish champions led three times in a pulsating 3-3 draw, has now put City firmly on the back foot in Group C with no more margin for error.

Anything but a defeat in Germany in two weeks’ time will ensure that City remain confident of progression, with Monchengladbach faced with the daunting prospect of a decider in the Nou Camp next month, but the 4-0 defeat against Luis Enrique’s team two weeks ago has dented City’s confidence and they are by no means safe bets to merge from this group.

Raheem Sterling congratulates Ilkay Gundogan on his first goal (Getty)

City’s problem is that they cannot defend properly at this level to have any hope of success.

Conceding three goals against Celtic was a warning sign of the frailties that Guardiola has been unable to address, but the roof caved in against Barcelona in Catalonia and it continued to leak profusely against the Spanish champions at the Etihad.

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 the rash left-back, Lucas Digne, but the England winger was let down by Hungarian referee Viktor Kassai, who failed to book Digne for either challenge.

An early Messi goal handed the visitors the lead (Getty)

Worse was to come for Sterling on 11 minutes, though, when he was booked for Kassai for diving when he had clearly been tripped in the penalty area by Samuel Umtiti.

City were incensed, and rightly so, but the incident affected their concentration and Barcelona took advantage in ruthless fashion ten minutes later.

That City conceded within ten seconds Sergio Aguero having a shot blocked at the other end was testament to Barca’s incredible counter-attacking.

With the ball dropping to Messi on the edge of the Barcelona penalty area, the Argentine sprayed a pass to Neymar before racing towards the City eighteen yard box.

Messi burst through the middle while Neymar ploughed down the left, all the time left unmarked by the home team.

And when he received a return ball from Neymar, Messi was allowed to run through a gaping hole in the centre of the City defence before beating Willy Caballero from ten yards.

It was too easy and Barca continued to pick City off following defensive mistakes until Gundogan hauled the hosts level on 37 minutes.

Sergio Busquets attempts to disposses Gundogan with a sliding challenge (Getty)

This time, a poor cross-field ball by Sergi Roberto was cut out by Aguero, who squared to Sterling. Sterling then cross to the far post for Gundogan to score a crucial equaliser.

City’s equaliser rattled Barcelona, however, 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.

But while the Belgian’s effort was deserved, City could and should have been even further ahead by the hour mark following the catalogue of chances created inside the opening quarter hour of the second-half.

Sterling was guilty of missing the first chance when, having been set free by Aguero, he failed to shoot first time and instead allowed a heavy touch to take the ball out of play.

De Bruyne then scored from 20 yards after David Silva had been bundled over just outside the penalty area, but the City onslaught continued.

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.

Kevin de Bruyne looks to get away from Sergi Roberto (Getty)

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 David 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 squad for Sergi 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 game.

But at 2-1, City remained vulnerable to a Barcelona counter until 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.