Police confirmed four arrests were made before kick-off while there were reports of scuffles inside the stadium. However, when the match did get under way, those travelling fans who got in were treated to a stunning 40-yard strike by Jhon Cordoba that put Cologne ahead with Arsenal goalkeeper David Ospina caught out of his area.
Arsenal improved in the second half and substitute Sead Kolasinac volleyed them level before a brilliant goal by Sanchez gave them a 67th-minute lead. The Chilean cut in from the left flank before curling a sublime effort into the far top corner, and the three points were secure when Hector Bellerin made it 3-1 late on after Theo Walcott's effort had been saved by Timo Horn.
Earlier, Everton's group-stage campaign got off to a disastrous start as Ronald Koeman's side were taken apart in Italy, losing 3-0 to Atalanta. Meanwhile, AC Milan underlined their status as one of the teams to beat in the competition by romping to a 5-1 victory at Austria Vienna in Group D with Andre Silva scoring a hat-trick.
Other winners on Thursday included Villarreal, who beat Astana 3-1 in Spain in Group A, and Real Sociedad, who crushed Rosenborg 4-0 in Group L. Mario Balotelli scored as Nice won 5-1 at Zulte-Waregem of Belgium in Group K, in which Lazio also won, 3-2 at Vitesse Arnhem. Marseille beat Konyaspor 1-0 in Group I while Athletic Bilbao drew 0-0 at Hertha Berlin in Group J.
Copyright Agence France-Presse, 2017