Arsenal fought the good fight, but ultimately capitulated to a superior Borussia Dortmund side at the Emirates Stadium.
It was the Gunners' first defeat since their humiliation way back on opening day against Aston Villa.
Arsenal and Dortmund matched each other's quality for stretches, but the Germans were the much more aggressive and robust side. Ultimately, the Gunners could not cope with the relentless pressure applied by Dortmund.
After a sterling run of form, Arsenal's momentum might be blunted a bit ahead of their most difficult stretch of the season. Let's look at six things we learned from the match.
Mesut Ozil was purchased for these sorts of games. Arsenal needed a truly elite attacking midfielder to give them an edge against some of the best teams in the world, and he did not provide it.
This was partially a result of his deployment on the left, and then right, wings for the entire game. Ozil can play there, but he is much more effective in the middle, where his creative talent flourishes. Arsene Wenger should have moved Tomas Rosicky over instead.
Still, Ozil was largely confined to simple passing and moving throughout the game, and could not find a way through the Dortmund players that mobbed him whenever he touched the ball. He flashed his quality on a couple occasions, but, overall, he did not live up to expectations.
Arsenal were never able to maintain possession in midfield—and certainly not in more advanced areas—for nearly as long as they do against lesser opponents. Every time the ball found its way to a player in a red shirt, at least two Dortmund players gravitated toward it.
As a result, the Gunners could not develop any sort of rhythm and were constantly straining to simply keep the ball away from their opponents. It's a strategy that is employed by the best, and fittest, sides in the world, such as Barcelona and Bayern Munich.
Watching Arsenal attempt to move the ball out of their defensive area and up the pitch was like watching a man who cannot swim trying to avoid drowning in the ocean. Dortmund kept up the pressure all game.
Aaron Ramsey knows that he is at fault for conceding Borussia Dortmund's first goal.
His boneheaded attempt to dribble the ball past multiple opponents on the edge of his own penalty area was reminiscent of Cesc Fabregas' inexplicable backheel pass against Barcelona two years ago.
The brazen move was likely borne out of the spectacular vein of form that Ramsey has hit recently. Everything has come off for the Welshman, including one of the best individual goals we have seen this season against Norwich last weekend.
After predictably being stripped of the ball, Henrik Mkhitaryan was perfectly placed to slot the ball past a helpless Wojciech Szczesny. Ramsey has hopefully learned a painful lesson.
Arsenal sorely lacked mettle in midfield and width on the wings.
Mikel Arteta was the only man who guarded the defense, and while he performed his task well, he operates better with an enforcer like Flamini beside him who is disciplined and practiced in the dark arts.
Further forward, Arsenal had almost no options on the wing when Borussia Dortmund inevitably pushed them out of their comfort zone in the middle of the pitch and forced the Gunners to rely on the flanks to create anything.
It was no surprise that Arsenal's only goal emanated from a cross, but it is equally telling that Bacary Sagna delivered it. He and Kieran Gibbs had no one in front of them.
For the most part, Arsenal were able to cope with Borussia Dortmund's many attacking threats. The Gunners' defending was a bit scrambled at times, but they can generally cope with technically skilled attackers.
Robert Lewandowski was an entirely different proposition.
Every time he sniffed the ball in or around the penalty box—and he threatened often—he would use his mighty strength to hold off two or even three Arsenal players at a time.
Yet what differentiates Lewandowski from similar players like Olivier Giroud is his quickness on the ball. His ability to flummox defenders with the sort of dribbling ability more commonly seen in petite wingers was truly remarkable and something that Arsenal never were able to counter.
At the moment, Arsenal still sit atop their group. But they are now tied with Dortmund and Napoli, who each have the same six points that the Gunners do.
And it only gets more difficult from here.
In a short time, Arsenal will travel to the fortress that is Signal Iduna Park in Dortmund, where they will need a victory—and absolutely must avoid a loss—to keep hope of topping the group alive.
Arsenal now have their first stern test in a season that has borne them nothing but fruit thus far. They must respond decisively, because there is a realistic possibility that in this, of all years, Arsene Wenger's side will not qualify for the Round of 16.