Arsenal have finally past their test. Wednesday's result cannot be diminished as the previous 3 or 4 tests have been; there is nothing that can be taken away from Arsenal, though some undoubtedly will try. Wednesday's result was built on the same foundations that has seen them win 16 of their last 20 Premier League games, drawing 3 and losing just the one. The turning point, of course, was replacing Thomas Vermaelen with Laurent Koscielny, and the team tightening up at the back. That saw Arsenal score less, but this season, when Arsenal have been more attacking, they've retained their shape without the ball. It's that retention of shape that saw Arsenal hold out Wednesday: Laurent Koscielny and Per Mertesacker were, again, excellent, and were well shielded by Aaron Ramsey and Mikel Arteta. This, again, isn't new: Arsenal's defence has been extremely sound since March. Of the games where both Koscielny and Mertesacker have both completed 90 minutes, Arsenal haven't lost since January 2012.
The aspect of play that Arsenal have added is ball retention as a defensive mechanism. In the past, Arsenal's ball retention was a defensive mechanism in that other teams didn't have the ball, but with Arsenal trying to attack, they focused far less on shape. Now, Arsenal will maintain possession in a less penetrating, but still effective way. Instead of committing numbers forward, they'll use possession to relieve pressure off of the defence, so even as Arsenal failed to have a shot in the first half, the fact that they had more possession than Dortmund wasn't a bad thing, as it lessened the pressure they were under. They did the same thing for various moments after they took the lead, and again, it took the sting out of any comeback Dortmund were going to mount. It's the sign of a team that's becoming increasingly mature: instead of trying to score more and more goals, Arsenal are more concerned with killing the game off.
Again, this isn't new for this season: Arsenal have played that sort of possession play at the end of victories against Napoli and Liverpool, as well as the run of victories at the beginning of the season, aside from the Tottenham match, where Arsenal were more resolute and completely focused on shape. The tactical variation that Arsenal has has been evident for most of this season: they can dominate possession, or play on the counter, with the latter allowing for control of possession as a defensive mechanism before quickly changing the gears and attacking space. Wednesday's performance confirmed this variation.
That Arsenal's goal was scored by Aaron Ramsey also isn't much of a surprise. The Welshman has always had a knack of getting into good scoring positions, but until this season, his finishing has shown a lack of confidence and composure. Now, full of confidence, Ramsey is finishing those chances, giving Arsenal's play another level of unpredictability: so often, his runs into the box are late enough that they're not tracked. He's also reveling in the space that Mesut Özil leaves for Ramsey, either by drawing markers away from him, or by drifting to the flanks, creating room for Ramsey to play in. While it's untrue to say that the difference between Arsenal this season and last season is Özil, it is fair to say that Özil has given the other Arsenal players more space and more freedom.
As Arsene Wenger said, Arsenal have only won two games this week; it is too early to definitively say Arsenal will win the title this season. There is, however, a sustained level of performance and tactical variation that makes it seem that this form will continue: Arsenal have take 2.49 points per match in their last 26 Premier League games, and are not just relying upon a single player to play well, as they might have in the past with Cesc Fabregas and Robin van Persie. Indeed, what is remarkable about this run of form is that, aside from Aaron Ramsey, there is no stand out brilliant performer: rather, everyone is contributing at a high level. Arsenal have been doing that for the past 7 months; Wednesday night was just another reaffirmation of what we have known for those 7 months.