Ciro Immobile opened the scoring after powering past Laurent Koscielny to beat Wojciech Szczesny with a low finish.Dortmund doubled their lead when Pierre-Emerick Aubameyang finished another fine breakaway.Aubameyang also hit the bar, while Danny Welbeck wasted Arsenal's best chance when clean through.Arsenal, competing at this stage of the Champions League for the 17th year in a row, have progressed to the knockout rounds before having losing their opening group game.But they will have to improve significantly after a woeful display in which they were outfought and outplayed for long periods at Signal Iduna Park.The Gunners arrived in Germany unbeaten in their previous six visits to the country.Yet Dortmund, who had 22 shots compared to Arsenal's four, should have won by a more handsome scoreline after a sloppy and careless performance by Arsene Wenger's side.The visitors, missing defenders Nacho Monreal, Mathieu Debuchy and Calum Chambers, fielded 19-year-old full-back Hector Bellerin, whose only previous senior outing was a 26-minute substitute appearance in a League Cup tie.Along with his team-mates, the Spaniard - who spent three months of last season on loan in the Championship at Watford - spent most of the first half on the back foot.Sokratis Papastathopoulos and Henrikh Mkhitaryan had already let the Gunners off the hook with some wasteful finishing before Immobile broke the deadlock in the 45th minute.It came after Arsenal conceded possession on the edge of the Dortmund area, a long clearance finding former Torino forward Immobile, who kept his composure before finding the net with a low angled finish after gliding past Koscielny.Shortly before Immobile's opener, Welbeck, on his second Gunners appearance since joining from Manchester United, dragged a wonderful chance wide from inside the area after clever play by Aaron Ramsey.It was to prove a costly miss.The second half was just three minutes old when Aubameyang skipped round Szczesny to bundle the ball over the line after the Gunners keeper and Per Mertesacker had got in each other's way.It could - and should - have been worse for Wenger and his men.Aubameyang rattled the bar with a rising shot from the edge of the area after Arsenal's defence was again hopelessly exposed by another swift German counter-attack.Defeat leaves the Gunners bottom of the group after Galatasaray and Anderlecht drew 1-1.(BBC)Bakudaily.Az