Match ends, West Bromwich Albion 3, Arsenal 1.
Arsenal suffered a fourth defeat in five league games as Craig Dawson's double helped West Brom inflict a major blow to the Gunners' hopes of a top-four finish.
Dawson capitalised on some awful defending from the Gunners to head both of his goals from corners - the first to give the Baggies the lead, the second to seal their win.
Arsenal had initially rallied after falling behind, levelling through Alexis Sanchez's close-range finish - the Chilean forward's 18th league goal of the season.
However, they fell behind again 10 minutes into the second half when substitute Hal Robson-Kanu rolled the ball home with only his second touch after coming on.
Arsenal, who lost goalkeeper Petr Cech to injury, were outfought and unable to rally a second time, their challenge wilting soon after Danny Welbeck's header from a corner struck the crossbar.
It was an eventful encounter, both on the pitch and above it as two planes pulling banners - one pro-Arsene Wenger and the other against the Arsenal boss - made circles around The Hawthorns.
It was the anti-Wenger camp who were most present and vocal by the end, though, holding aloft banners declaring 'Enough is enough' and booing their beleaguered manager's decision to substitute Sanchez, who had picked up a knock.
After the game, Wenger said that he would announce "very soon" whether he will remain at the club after reaching a decision on his future.
The current facts are these - Arsenal trail fourth place by five points (albeit with a game in hand).
They must arrest a run that has seen them win just one of their last five league games if they are to continue their proud record of finishing in the top four in each of Wenger's 20 seasons at the club so far.
The end is nigh?
After claiming a top-two finish in every season between 1997-98 and 2004-05, Arsenal have often flirted with fifth or worse since, but this is looking increasingly likely to be the campaign when they drop below the division's premier quartet.
There is also a growing feeling this is likely to be Wenger's last as manager.
A two-year contract extension remains on the table but with fan protests growing in size and volume, rumours of players wanting away from the club and form deteriorating, there may be no appetite for it to be signed, by either manager or club.
This result felt like a new low during Wenger's tenure.
The Gunners boss is used to seeing his side outfought and beaten at the home of teams managed by Tony Pulis - he has only managed one win in eight such encounters - but the lack of chances they created from almost 77% possession and the speed with which they fell apart after Robson-Kanu's goal, lays bare a side with dwindling belief and backbone.
They rallied efficiently after falling behind, but this also speaks to their over-reliance on Sanchez, whose movement made the goal and who remained their only real creative spark, despite the heavy-handed treatment dealt out to him by the home side.
Their vulnerability at the back is alarming. They knew about West Brom's threat from corners - the Baggies have now scored 14 from them this season - but yet they were unable to prevent Dawson twice getting his head to the ball first in the box to score.
Robson-Kanu's goal was equally damning as Ospina proved an inadequate replacement for Cech, sliding out to meet a chipped ball over the top and gifting the striker his finish.
West Brom corner poor Arsenal
West Brom have never managed more than 49 Premier League points. Prior to Saturday they had not managed to beat a top-seven team this season.
Having rectified the latter, they now look on course to better the former.
They came into this game having lost their last two games and from the off it was clear they were not prepared to entertain the prospect of suffering three successive defeats for the first time this campaign.
But for Cech, they could have been ahead before Dawson broke the deadlock, with the Gunners keeper tipping away Darren Fletcher's goalbound drive.
When they were pegged back by Sanchez, their belief held, as did their faith in Pulis' game plan, which also stretched to superbly-timed substitutions.
It helps, of course, when you are so lethal from set-pieces and your opponent so incapable of defending them.
Man of the match - Craig Dawson (West Brom)
'We face some serious challenges' - What the managers said
West Brom manager Tony Pulis, after ending a run of two defeats: "I think everybody's got to calm down. You're always going to have runs of games where you don't pick points up. You raise the expectation and people get carried away.
"The lads have worked so hard in all the games. I thought we'd never play as well as what we have played. They've never given anything but their best.
"The lads were a bit disappointed with two defeats. They've done fantastically well. They've carried on working so hard together as a group. I thought on the counter we always looked dangerous.
"We've got 43 points. We know where we are as a football club. We're not getting carried away.
"We'll do the best we possibly can. If we win games, brilliant. If we don't we've still got a group of honest players who give everything."
Arsenal manager Arsene Wenger: "It was a typical Premier League game. A team that likes to play and a team that defends well.
"It was a tough performance. They caught us on set-pieces and one break and that made the difference.
"Overall our record against set-pieces is quite good. We were a bit naive, maybe, on the corners. Then we were punished. It's a shame. We looked in the second half to take completely over.
"We didn't create enough. We lost Sanchez in the second half, he was very dangerous in the first. He came out in the second half and he couldn't move any more. In the first half he was a guy who created a lot.
"It leaves us in a unique situation that we've never had before. We face big problems to regroup and find resources to sort out the problem. We need some togetherness.
"We face some serious challenges. The City game at home is a big game for us."
Sanchez hits 100 - the stats you need to know
- Arsenal have lost four out of five Premier League games for the first time in since April 1995.
- Alexis Sanchez has been involved in 27 PL goals this season (18 goals, nine assists) - more than any other player in the competition.
- Overall, Sanchez has now been involved in 100 goals for the Gunners in all competitions, scoring 64 and assisting 36.
- There were just two minutes and 45 seconds between the first two goals in this game.
The international break means the Gunners host Manchester City on Sunday, 2 April.
West Brom take on the other Manchester side that weekend, with United hosting the Baggies on Saturday, 1 April.
- 22ChadliSubstituted forFieldat 90'minutes
- 11BruntSubstituted forYacobat 72'minutes
- 14McCleanBooked at 42mins
- 9RondónSubstituted forRobson-Kanuat 54'minutes
- 33CechSubstituted forOspinaat 38'minutes
- 14WalcottSubstituted forGiroudat 65'minutes
- 7SánchezSubstituted forIwobiat 78'minutes
- Neil Swarbrick
- Shots on Target
Second Half ends, West Bromwich Albion 3, Arsenal 1.
Attempt missed. Aaron Ramsey (Arsenal) right footed shot from outside the box is high and wide to the right. Assisted by Danny Welbeck.
Corner, Arsenal. Conceded by Jonny Evans.
Substitution, West Bromwich Albion. Sam Field replaces Nacer Chadli.
Attempt blocked. Aaron Ramsey (Arsenal) right footed shot from outside the box is blocked. Assisted by Alex Oxlade-Chamberlain.
Foul by Aaron Ramsey (Arsenal).
Nyom (West Bromwich Albion) wins a free kick on the left wing.
Foul by Danny Welbeck (Arsenal).
Jake Livermore (West Bromwich Albion) wins a free kick in the defensive half.
Attempt missed. Aaron Ramsey (Arsenal) header from the centre of the box misses to the left. Assisted by Shkodran Mustafi with a cross.
Substitution, Arsenal. Alex Iwobi replaces Alexis Sánchez.
Corner, Arsenal. Conceded by Nacer Chadli.
Attempt blocked. Nacho Monreal (Arsenal) header from the left side of the six yard box is blocked. Assisted by Aaron Ramsey with a cross.
Goal! West Bromwich Albion 3, Arsenal 1. Craig Dawson (West Bromwich Albion) header from very close range to the high centre of the goal. Assisted by James McClean with a cross following a corner.
Corner, West Bromwich Albion. Conceded by Shkodran Mustafi.
Substitution, West Bromwich Albion. Claudio Yacob replaces Chris Brunt.
Attempt blocked. Granit Xhaka (Arsenal) left footed shot from outside the box is blocked.
Danny Welbeck (Arsenal) wins a free kick in the attacking half.
Foul by Chris Brunt (West Bromwich Albion).