By Josh James at The Hawthorns
Arsenal were heavily punished for defensive failings at set pieces as West Brom inflicted a damaging defeat.
Craig Dawson thumped home headers from corners either side of the break, while Hal Robson-Kanu capitalised on a goalkeeping error in between those goals.
Alexis had equalised just two minutes after the hosts had taken the lead in the opening half, but that was as good as it got for us on a forgettable afternoon.
Petr Cech was taken off due to injury late in the first half - shortly after saving well from Darren Fletcher - to compound our misery.
It means we face an uphill challenge to break back into the Champions League places.
SETTING THE SCENE
After greatly contrasting fortunes in the last two matches - in the Champions League and FA Cup - focus returned to the Premier League and the race for the top four.
The Gunners travelled to The Hawthorns for the early Saturday kick off looking to make up ground on the teams above us, having lost three of our last four league games.
Last weekend's comfortable win over Lincoln served to restore confidence though, and the manager chose to stick with largely the same starting XI.
The only two changes were Nacho Monreal for Kieran Gibbs at left back, and Danny Welbeck for Olivier Giroud up front.
Aaron Ramsey kept his place to make his 200th appearance in the Premier League.
Mesut Ozil was missing with a light hamstring strain, but Mohamed Elneny recovered swiftly from his ankle injury to be named on the bench.
West Brom had lost their last two games, each without scoring, but won this fixture last season - their only victory over us in the past 12 meetings.
Allan Nyom's marauding run down the left wing was the first scare for the Gunners. The full back broke past two or three tackles on his way to the penalty area, where he was unable to pick out a team mate with his low cross.
We also threatened on our left flank. Alexis fed Monreal inside the area, but his pass to the near post was scrambled away to safety.
Another break down the West Brom left led to the first set piece of the game, and the hosts took full advantage.
Nacer Chadli swung in a corner to the near post where Dawson rose highest and nodded home.
The lead lasted just two minutes though. Granit Xhaka had time to direct a pass into the area, which Alexis controlled on his chest, took a touch, then rifled home from six yards.
The game was being played at a hectic pace, with space appearing at both ends of the pitch.
West Brom looked dangerous in wide areas especially, and Chadli sprung free on the right to send Rondon away, only for the Venezuelan to drag his shot wide.
At the other end Ramsey tested Ben Foster with a low shot, and Walcott was unable to react quickly enough to the rebound.
Cech then superbly denied Fletcher, but it was the keeper's last involvement, as he replaced moments later by David Ospina after picking up an injury.
Late in the half James McClean became the first player to go into the book after he chopped down Alexis – the Chilean had been on the receiving end of a number of hefty challenges.
Rondon headed an early chance wide, but it was his replacement Robson-Kanu who gave West Brom the lead.
Ospina and Hector Bellerin got in a mess defensively, the ball bounced off the 'keeper's legs and the Welsh striker had the simplest task of tapping home.
We looked rattled, but moments later it was the frame of the West Brom goal which rattled, when Welbeck powered a header against the bar from an excellent Xhaka corner.
As we pushed forward, gaps appeared at the back. Robson-Kanu was denied by the advancing Ospina, and Shkodran Mustafi was the saviour when Chadli sent the loose ball goalwards.
Somehow Arsenal survived.
We couldn't survive another aerial assault from a corner though, as Dawson netted his second of the afternoon. The unmarked defender soared highest in the area and thumped his header home from McLean's corner.
The third goal was a hammer blow, and Arsenal's play became ragged and disjointed for the final 15 minutes or so, as Foster remained untroubled in the hosts' goal.
Referee: Neil Swarbrick
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