There were several key factor to Arsenal's Champions League defeat of Barcelona, only the Spanish side's second loss this season. Andy Mitten, European football correspondent, analyses Arsene Wenger's tactics.
How Arsenal beat Barcelona
Arsenal did it. They beat a Barcelona team regarded by many as the best ever. A line-up packed with World Cup winners and Spanish champions, footballers who have been continually showered with the game's highest individual honours.
When David Villa put Barca ahead in London after 26 minutes, everything was going to plan for Pep Guardiola's dominant side. But two brilliantly taken goals in the final 12 minutes saw Arsenal triumph 2-1. Barca's Xavi claimed the difference was "Arsenal being effective in front of goal and we weren't" - but there was more to Arsenal's victory than that.
"They position themselves well and they are very good on the counter attack," said a magnanimous Guardiola after the game. "They are very quick at using space."
When Barca lose the ball in most domestic matches, they bust a gut to win it back. Opponents are usually playing so deep that Barca regain possession in the half they are attacking.
Arsenal played a high defensive line and despite having less possession, they moved the ball quickly when they won it. The speed at which they did hurt Barca. And because Barca play such a high defensive line themselves, there was space to be exploited.
Messi's English drought
The world's best player burst into British consciousness at Stamford Bridge in 2006. He has had some good performances in England, but despite averaging more than a goal a game in Spain, Messi has not scored in seven games in England with Barca.
At the Emirates Stadium on Wednesday, he had a goal wrongly disallowed for offside. Arsenal needed luck to beat Barca and they got it there.
Barca's respect for Arsenal was genuine, but Arsenal appeared overawed by their big name visitors when the sides met last year. Not this time.
Arsenal's never-say-die attitude prevailed with two late goals to win the game - or "Six Fateful Minutes" according to the headline on one Catalan daily.
Having seen his side get outplayed and fall behind to Barca's incisive first-half passing, Arsene Wenger boldly introduced the attacking Andrey Arshavin and Nicklas Bendtner.
At the same time, Guardiola made the cautious introduction of Seydou Keita, the defensive midfielder, for Villa.
Fortune favoured the brave as Arsenal's more attacking formation created the opportunities which would lead to two goals - the winner from his Russian substitute Arshavin following Robin van Persie's equaliser. Suddenly a brilliant Barca looked vulnerable.
The tie is not over
Barca's away form in Europe is far from spectacular and they draw more Champions League away games than they win.
They drew away at Rubin Kazan and Copenhagen this season, before winning both home ties comfortably 2-0.
In the knockout stages last year, they drew in Stuttgart and at Arsenal, before putting four past both sides at Camp Nou to progress. Their away goal means that they only have to win 1-0.
"It will be a spectacular return match," Guardiola said. A 98,000 crowd and 300 million watching on television relish the prospect.