Si se puede: How belief, and some luck, helped Barcelona make history

"Si se puede," crowed the audience at the Camp Nou. Yes we can. Barack Obama’s message of unifying optimism might have lost its power in the world of American politics, but inside a packed stadium of Catalan football fans it still felt as vivid as ever.

When you have spent the last decade watching Leo Messi make magic on a weekly basis, the word "impossible" rather loses its meaning.

Make no mistake, Barcelona’s task should have been impossible. Down 4-0 from the first leg of the last-16 tie against Paris Saint-Germain, Barca required a comeback on a scale no team in Champions League history had ever achieved. This against an opponent that had not lost a game by four goals in seven years. An opponents that played Barcelona off the pitch at the Parc des Princes.

And yet, the Blaugrana still believed. At his pre-game press conference, Luis Enrique opined that: "If a team can score four against us, we can score six."

How many people truly believed him in that moment? How many still believed in the 88th minute on Wednesday night, when Barcelona found itself only half-way to that target? Edinson Cavani’s away goal meant Barca would need all six to progress.

Even after Neymar had whipped a perfect free-kick into the top corner of the net for Barcelona’s fourth, it still seemed an unreachable goal. But then Luis Suarez took a tumble in the area - and it must be said that this looked like a dive - and won a penalty. Neymar converted that, too, setting up the grandest of grandstand finishes.

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Of all the players to complete this comeback, Sergi Roberto might have been the most improbable. Repeatedly targeted and horribly exposed during the first leg, he had been subsequently been dropped from Enrique’s starting XI - his inadequacy at right-back helping to convince the manager to reshuffle his team into a 3-4-3.

Roberto entered this game only as a 76th minute substitute, and playing in midfield. But he would be the man who settled this tie, sprinting onto Neymar’s chipped ball over the defence and volleying beyond Kevin Trapp.

The wild eyes of Barcelona’s players betrayed the reality that even they were finding this a little hard to believe. They had pulled off not only the greatest escape in Champions League history but one of the greatest in recent sporting memory as well. The New England Patriots' 25-point comeback in Super Bowl LI no longer feels quite so unique.

It will be fascinating to revisit this game once the dust has settled and emotions are running a little less high. The honest truth is that Barcelona - while drastically improved from their woeful first leg performance - did not feel all that irresistible at the start.

This new formation certainly suited the Spanish juggernaut better. Leo Messi was now spending more time in central positions, from where he could pull the strings. The host finished the first half with 64 percent of possession, and yet had not carved out all that many clear-cut chances. The two first-half goals owed more to calamitous defending than any great artistry or invention.

The second, indeed, was not even scored by a Barcelona player - arriving instead via the boot of PSG’s Layvin Kurzawa, who sliced a panicked clearance into his own net. If that was an act of self-harm, then even more damaging to the French side was manager Unai Emery’s decision to instruct his team to sit back and defend deep.

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The Ligue 1 champion has thrived in the first leg by assaulting Barcelona with a high press and a relentlessly aggressive tempo. The newfound passivity did not suit the French side nearly so well.

Even so, when Cavani blasted the ball beyond Marc-Andre Ter Stegen just after the hour mark, PSG seemed to have done enough. That this turned out not to be the case speaks above all to Barcelona’s renewed competitive spirit.

The side that lost in Paris was listless, sluggish, resigned. But that was before Enrique promised to resign at the season's end.

Whether players were gratified by that decision, or simply shaken back to their senses by it, the fact remains that in the three games since the start of this month, Barcelona has scored 17 times and conceded just twice.

Related: Did Luis Suarez dive to win crucial late penalty for Barcelona?

Barca got lucky against PSG. Had Suarez been penalised for diving in the 90th minute, instead of being rewarded with a penalty, then this miracle would never have come to completion. And yet, if the players had not believed then they would never have put themselves in position to benefit from such fortune in the first place.

"Si se puede." Yes we can. Yes they did.