Arsenal endured a crushing defeat in their Champions League Group C opener. Despite taking a first-half lead through Mariona Caldentey, Jonas Eidevall’s side were beaten 5-2 by an energised Bayern Munich. Pernille Harder stunned Arsenal’s defence with a 13-minute hat-trick after the two sides had traded goals from Bayern’s Sydney Lohmann and Arsenal’s Laia Codina. It may seem excessive at this early stage to think that this week could be a defining one in Arsenal’s season. Once again, however, there is trouble afoot in north London and there was no doubt going into this fixture how significant it would be before ­Chelsea’s visit in the Women’s Super League on Saturday. The ­manner of this defeat and the way Arsenal conceded the last three goals will have been particularly galling for Eidevall as pressure continues to mount. “We have no choice but to put this game here behind us – take the ­necessary learnings from it and get ready for the task on Saturday,” the Arsenal manager said. “The first half was a good half of football for us. Maybe we were a bit unlucky having the game tied 1-1. ­Second half, I think Bayern upped their intensity. They threatened us much more with runs in behind and asked us more difficult questions. We got a little bit stretched and tired, and a little bit hesitant in our pressing. What really strikes in the second half is how we defended crosses and that is not good enough.” An underwhelming draw against a struggling Everton side last Sunday has left Arsenal trailing domestically and there is unquestionably high expectation on the team to progress in Europe after the ignominy of being absent last season. It will certainly not be an easy task – particularly after this result – with Arsenal drawn in a challenging group. A trip to Munich was the first hurdle against a familiar opponent. Their more recent domestic struggles prompted Eidevall to make five changes to his starting lineup. Manuela Zinsberger, Leah ­Williamson, Caitlin Foord, Lia Wälti and Stina Blackstenius all returned with the intention to add directness as well as some much-needed experience. In opposition, there were ­familiar faces in Alexander Straus’s side. Harder faced Arsenal many times during her Chelsea years as did ­­Georgia Stanway in her time at ­Manchester City. Bayern came into this match in flying form – they have won their past seven in all competitions and have been beaten only once in their past 17 home Champions League games. Eidevall was right to be pleased with the way his side started. The manager had issued a rallying cry for his side to “go full throttle” and his players responded. Arsenal were the better team for the majority of the first half as they pressed with intensity. The reward for their efforts came after half an hour when ­Mariona deftly met Katie McCabe’s delivery to volley past Maria-Luisa Grohs. Their joy was short-lived, as Straus’s team sensed Arsenal were letting their foot off the gas. A sustained period of pressure ended in an equaliser for the hosts: Glodis ­Viggósdóttir, the captain, rose high in the box to head home just before half-time. The hosts returned to the field a different beast as they looked to stretch the creaking Arsenal back line with their pace. It was a policy that paid off when Lohmann scampered into space down the right before squeezing a shot past Zinsberger at the near post. The visitors rallied briefly with Codina heading them level once more but, down the other end, everything fell apart. Harder, the trailblazing Danish midfielder, took responsi­bility as she so often has in her 50 Champions League appearance to date. She marked her 50th in style with a quickfire treble that left the Arsenal players and bench stunned. Three deliveries into the box, three instances of ineffectual defending that a player of Harder’s quality will almost certainly pounce on. She rose high to head in two corner deliveries before reacting quickest to her own parried shot to ensure that Bayern’s comeback was complete. It was a bitter pill to swallow for Arsenal. Eidevall now potentially has the two biggest days of his tenure to find a way to rally his side before a huge London derby. Defeat must be avoided if they are to have a chance at getting their season back on track.