Bayern Munich overcame an early scare to emerge victorious from the Emirates Stadium on Wednesday night.
Arsenal started strongly with Mesut Ozil missing a penalty inside the first 10 minutes, then Wojciech Szczesny was sent off for lunging at Arjen Robben in the box. David Alaba missed the follow-up spot-kick, but Toni Kroos broke the deadlock soon after the second half kicked off.
Thomas Mueller, a second-half substitute, put the icing on the cake with a beautiful late header after Philipp Lahm's inch-perfect cross.
Let's take a look at how the game played out from a tactical angle.
Arsenal set up in their regular 4-2-3-1 formation, with Yaya Sanogo drafted in for a surprise start in the centre-forward role and Alex Oxlade-Chamberlain on the right-hand side. Mathieu Flamini marshalled a cautious defensive midfield, while Kieran Gibbs was selected over Nacho Monreal at left-back.
Bayern Munich moved Lahm to right-back and drafted Javi Martinez into defensive midfield in a standard, flat 4-1-4-1. Arjen Robben began from the left, Mario Goetze from the right.
Energy & Tempo
Tactically speaking, this didn't start well for Bayern.
Arsenal floored the accelerator from the first whistle, and their raw energy and gusto had the visitors shell-shocked. It's becoming more common to see Bundesliga teams "give up" before the game has even started when die Bayern come to town, and it was evident they hadn't played a truly intense 90 minutes for weeks.
Oxlade-Chamberlain led the charge, and the midfield's strong collective pressing forced several early mistakes from Martinez and Dante in the defensive third.
Pep Guardiola had clearly instructed Dante to aggressively pursue the centre-forward—perhaps expecting the industrious, wandering Olivier Giroud to start—and he followed the same brief with Sanogo.
The Frenchman dropped deep and linked with his midfield, and Dante was happy to close him down with Martinez dropping back into the space left behind. It was similar, in a way, to Martin Demichelis's pursuit of Lionel Messi with Vincent Kompany and Fernandinho sweeping behind.
Martinez has only just returned from injury, and both his instincts and touch were off the pace. Couple that with the tempo Arsenal set, and it made for a cacophony of early errors.
Flamini Stops Left-Sided Overload
Eventually Bayern found their feet, with Robben getting more involved, Thiago finding the ball often and, critically, Lahm steaming forward from right-back.
Die Roten spent a good 30 minutes attempting to overload the right side with players and jinking runs, with Lahm's impeccable decision-making—does he ever lose the ball?—a key part in sustaining pressure in advanced zones.
Goetze was key with his movement, but they didn't manage to fashion much in terms of chances on goal. Why? Flamini, and to an extent Jack Wilshere, did a particularly excellent job sliding across to double up and take them on man-for-man on the edge of the box.
Flamini's clever positioning and unusually reserved tackling halted most of the action on that flank.
Just before half-time, Szczesny was sent off and Alaba missed the resulting penalty. Wenger switched to a 4-2-3-esque shape with Sanogo off the front line at the break, while Pep took off Jerome Boateng (CB) for Rafinha.
The Brazilian took his regular place at right-back, Lahm moved into midfield and Martinez joined Dante at the back. The formation changed subtly, too, with the shape approaching—but perhaps not entirely reaching—a 4-2-3-1, with Thiago further forward and Lahm partnering Kroos in holding midfield.
This paved the way for dominance of the ball for the visitors, and Arsenal singularly failed to break away from their defensive structure. Transitions were essentially impossible given the amount of physical work they had to do to get close to the opposing half.
The breakthrough goal was absolutely splendid, but it came via the route Flamini had done so well to close off for most of the game: Bayern built up play down the right and laid a simple ball across the edge of the box, and Kroos obliged by curling a scintillating shot into the top right corner.
Two things were key here: Kroos hit it first time, giving Flamini and Wilshere no time to close the space between themselves and the ball, and Rafinha's runs down the right were far more aggressive and committed more defenders in deeper areas; it created room for the shot that wasn't previously there.
Bayern See It Out
Bayern finished with 79 percent of the possession, per WhoScored?, and looked far too accomplished to make a mistake.
The midfield three were working the ball around with ease, and while it's obviously a tough ask to halt this team in full flow, Sanogo's deployment in the second half was questionable.
He dropped deeper and deeper to try and cut out the passing angles but had no joy whatsoever. It meant the Gunners had no out-ball—they ended up in a very, very deep 4-4-1—and failed to climb out of their own third with regularity.
Sanogo could have been deployed in a suffoco-esque role—like Mario Gomez, Toni Kroos, Thomas Mueller and Mario Mandzukic were last season—in order to at least try and stifle the free possession.
Had he sat on the deepest midfielder, he would have had a more advanced starting position and could have disrupted the flow of passing a little.
The fact that Mesut Ozil remained on the pitch for the full 90 minutes was also a head scratcher. With Arsenal buried deep, they needed pace, power, hard yards and defensive nous; the German offered none of that and hindered his team far more than he helped them on the night.
Mueller's late goal effectively sealed the tie, and it looks set to be a second consecutive year of "what if?" questions running through Arsenal fans' minds.
Tactical Man of the Match: Philipp Lahm