The Polish champions were on Wednesday back on European club football's greatest stage for the first time since 1995/96 having negotiated three qualifying rounds. However, Besnik Hasi's side may not yet feel like they belong there, after conceding three goals inside 17 minutes on their comeback appearance.
First Mario G?tze nodded Ousmane Dembele's left-wing cross past Arkadiusz Malarz. Then, from one Rapha?l Guerreiro free-kick, Sokratis Papastathopoulos headed the second; and following the Portuguese's next set piece, Marc Bartra finished off a goalmouth scramble for 3-0
Guerreiro, operating in central midfield rather than on the left, swept home the fourth soon after half-time. Substitute Gonzalo Castro later turned in Christian Pulisic's centre, before Pierre-Emerick Aubameyang completed the rout on the counter.
Borussia Dortmund delivered an emphatic tactical and footballing display to mark a successful return to the Champions League, recording their highest-ever victory in the competition in the process, as they look ahead to the Group F showdown with Real Madrid in two weeks.
The match was overshadowed by the Legia Warsaw ultras attacking the 1,800 Black & Yellow fans, with the police intervening with pepper spray.
But Thomas Tuchel's new-look starting XI, with a 4-1-4-1 formation, hardly seemed fazed by the extraordinarily feisty atmosphere in the Polish capital, keeping the ball moving and their opponents chasing.
Shortly after Pulisic had threatened the hosts with a low cross into the penalty area, the visitors opened the scoring. Schmelzer played a long ball to Demb?l? on the left wing, the Frenchman crossing the ball in for G?tze to break the deadlock in the seventh minute.
It was the forward's fifth European goal for Dortmund, and his first since March 5, 2013.
Excellent build-up play four minutes later presented Aubameyang with a chance to double the lead, but he was twice denied by goalkeeper Malarz.
The shot-stopper was unable to keep out Sokratis moments later, however, when the Greek headed home Guerreiro's free-kick in the 15th minute.
Barely 60 seconds later the custodian saw yellow for handling outside the box. And although he was initially able to fend off the ensuing free-kick -- a well-placed effort from Guerreiro from 18 metres out -- and Schmelzer's attempt after Aubameyang's initial follow-up had rebounded off the post and into the German full-back's path, Bartra found the net at the third time of asking.
Dembel?, Sokratis, and Aubameyang all had their chances to score more in the rest moments of the second half but the first half ended 3-0.
Dortmund continued to dominate the second period without overexerting themselves. Roman B?rki, who had a quiet evening between the sticks, wasn't tested until Steeven Langil's effort in the 57th minute.
By then it was already 4-0. Five minutes after the restart, Aubameyang had cut in from the right and played a pass across to the unmarked Demb?l? on the right flank.
The latter saw his shot blocked as two defenders threw themselves into its path, but Guerreiro pounced on the loose ball and fired it into the far corner. A couple of efforts from G?tze shortly thereafter posed a threat but were ultimately blocked in the 57th and 59th minutes.
With a quarter of an hour on the clock, G?tze, Demb?l? and Julian Weigl were substituted. Substitute Castro needed just 30 seconds on the pitch before he added a fifth, pouncing at the far post to convert a Pulisic cross.
Then, in the 87th minute, Aubameyang finally got on the scoresheet after Castro had played the ball perfectly into his path to complete the rout of Legia.
