- Barcelona talisman Lionel Messi fired his side ahead coolly around Claudio Bravo on 17 minutes
- Claudio Bravo was sent off for handling outside his penalty area early on in the second half
- Messi made Manchester City soon pay for Bravo's dismissal as he grabbed his second goal of the game
- The Argentine completed his hat-trick on 69 minutes with a close-range finish at the Nou Camp
- Barca substitute Jeremy Mathieu was sent off for two bookable offences in the Group C encounter
- Neymar missed a penalty late on but quickly made amends with a neat finish to complete the scoreline
After the second of Lionel Messi's three goals went in, Pep Guardiola ran his hand anxiously across the top of his head. The Manchester City coach looked like a man who was watching his house burn down fully aware that it was he who had left the toaster on.
This, for all Messi's enduring and captivating brilliance, was defeat by managerial error.
City, as Guardiola had hoped, proved a match for Barcelona at times here. It seems strange to say given the score line but City began each half the better and they created several clear chances.
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In their previous visits to the Nou Camp in the Champions League they had not managed to do this. For the first time, this was competitive. It was, for example, only 1-0 to Barcelona at half-time and Luis Enrique had already lost two of his back four to injury.
Ultimately, though, decisions cost City and they were Guardiola decisions.
As a direct result of them, City ended this night thoroughly embarrassed as their run without a win stretched to four games
Strangely, Sergio Aguero was not selected here.
Guardiola explained he wanted strength in his midfield and the football sages no doubt nodded earnestly. But at times football is a startlingly simple game.
The City manager had said he wanted to win this match so surely his team needed a proper centre forward, still needed their best player. As a result of his exclusion, the Premier League team lacked the necessary expertise when their chances arrived.
Interestingly, Aguero was not on the field during the pre-match warm up. He did jog down the line early in the second half but was not brought on. Make of all that what you will.
And what now, meanwhile, of Guardiola's decision to jettison Joe Hart back in August?
Claudio Bravo, a goalkeeper purchased for his expertise with his feet, did on Wednesday night what he has done consistently since arriving in Manchester. He made a mistake with his feet.
In presenting the ball to Luis Suarez in the 53rd minute, Bravo was forced to save the resulting shot and was sent off for handling outside the penalty area.
It was a horror moment and it sunk City. Already a goal down to Messi's early strike, City were nevertheless well in the game at that moment but within minutes of Bravo's exit Messi scored again. Then, seven minutes later, he scored again.
A contest had become a walkover in a matter of minutes and Guardiola's return to his home had turned in to an exercise in self-destruction.
Managers are paid to make big decisions, of course, and occasionally they go wrong. But it is impossible to ignore Guardiola's decisively wretched influence on this game.
Early on, there seemed little alarm. The Aguero question hung in the air from the moment the team sheets dropped but with Kevin De Bruyne asked to operate at the top of City's formation there was a briskness about the visiting team.
Barcelona were not initially at their best. They were sloppy in possession and provided space for City's forward players to operate in. Raheem Sterling was progressive down the right while Ilkay Gundogan was bright in the centre of the field.
There was no Aguero, though. There was no real focal point and after City failed to turn early superiority in to goals, they handed Barcelona the lead.
Messi's first goal was a poor one. But then others were too.
The Barca No 10 had bustled feverishly down the right in the 17th minute but the move broke down when Pablo Zabaleta tacked Andres Iniesta. As the ball ran free in to the penalty area, either Fernandinho or Nicolas Otamendi could have cleared but one fell over, the other was caught on his heels and Messi eased past Bravo to slide the ball in to an empty goal.
It was a blow for City and for a while Barcelona had them on a piece of string. But it needn't have been fatal as eventually City rallied and ended the first half unfortunate not to be level.
Gundogan was denied by a brilliant Marc-Andre ter Stegen save after a dazzling run and then John Stones, unmarked, headed wastefully wide from a corner.
So at the start of the second half, as De Bruyne twice reached the byline to cross dangerously, City were very much alive. By now, Barcelona had lost Gerard Pique and Jordi Alba to injury.
Time for Aguero? Apparently not and as he watched on from the bench Bravo dashed from his goal to ruin City's evening.
Credit must go to Suarez. The cause he chased was a lost one but he chased it anyway because that's what he does. Had he not been in Bravo's vision, the clearance would have been less hurried. It was still a horrific goalkeeping error, though, and it ended the game.
Messi bamboozled Aleksandar Kolarov to score from 18 yards after Fernandinho was hustled out of possession just after the hour and then slid in his hat-trick goal in the 68th minute after a stray back pass from the hitherto impressive Gundogan played Suarez clear down the left.
Guardiola was hunkered down in his seat by now but will still have seen Messi leave Gael Clichy and Kolarov in a heap as he won a penalty that Neymar allowed substitute Willy Caballero to save. Two minutes later, though, the Brazilian slithered past Otamendi and Stones to make amends and score the fourth.
That was a sumptuous goal but it was merely a knife twist in a wound of Guardiola's own making. Is it possible to be too clever for your own good in football? Perhaps.