The Gunners were outplayed and outclassed in Munich, as they slumped to their third Champions League defeat in four outings this season

Bayern Munich romped past Arsenal as a breathtaking first-half display saw the Gunners' Champions League hopes dwindle once again.

Robert Lewandowski broke the offside trap to open the scoring, before Thomas Muller bagged two and David Alaba and Arjen Robben followed suit.

It was very much a men against boys performance in the first hour of the match, as Arsenal slumped to their third Champions League defeat in four outings this season.

They will now be required to pick up wins against Dinamo Zagreb and Olympiacos to secure their qualification for the knockout stages.

Here is what we learned from the game. . .

Sleepy Arsenal are back to old habits

Arsenal were absolutely magnificent when these two teams first met. On the ball they were clinical and off it they ran their socks off.

It was clear from the first whistle that we were not going to be watching the same Arsenal side. The Gunners were far more reluctant to put pressure on the ball and as a result, went behind within 10 minutes.

Although the finger could be pointed at Gabriel for the first goal, Arsene Wenger will no doubt be furious at the amount of time Tiago had to pick out a perfectly-placed cross for Lewandowski to nod home.

Unfortunately, that was a sign of things to come. Tiago and Xabi Alonso enjoyed a field day in the middle of the park. Loads of time to play with minimal pressure on the ball. They ran the show.

By the time the game entered the 30th minute, Bayern were literally walking the ball into the box.

Arsenal were playing with a death wish and got the tonking their work rate deserved. It was only in the final 25 minutes, when Bayern slowed down, that Arsenal looked like any kind of a threat.

Arsenal's squad cannot cope with injuries

Tonight, it was Laurent Koscielny's absence that took the pre-match headlines, after pulling out at the last moment with a hip problem.

But Wenger has a track record of continually relying on players with a history of problems. The squad, quite simply, is not strong enough to compete with the very best in Europe when they so regularly have so many players out injured.

Just compare the benches of the two sides: Arsenal called up the likes of Alex Iwobi and Jeff Reine Adelaide, while Bayern could turn to Arjen Robben, Arturo Vidal and Rafinha.

Within seconds of coming on, Robben had bagged Bayern's fourth goal of the game.

After going behind, Arsenal had nobody in their team who they could turn to and say 'change this game'. Bayern had already pressed Sanchez and Ozil out of the game.

Wenger's faith in Campbell backfires

It may have taken 18 months to show it, but Arsene Wenger really does have faith in Joel Campbell.

Just months ago, Campbell was very much considered an outcast at the Emirates, but Wenger's rotation system and a couple of injuries have paved the way for the Costa Rican to earn his chance.

Wenger has claimed that he has always had believed in Campbell - and he was handed a vital Champions League start at the Allianz Arena days after his first Premier League goal.

He was perhaps deployed because Wenger thought Campbell would be more effective working with Mathieu Debuchy down the right side to hold off the threat of Douglas Costa than Alexis Sanchez would have been had he switched flanks.

It seemed to be a gamble which backfired. Campbell struggled on the ball and looked out of his depth against the German champs.

The lack of Theo Wacott and Alex Oxlade-Chamberlain was evident as Arsenal failed to offer a threat down the right wing - both offensively and defensively.

Campbell was hauled off on the hour mark having made no impact on the game.

There is light at the end of the tunnel

Until the hour mark - when they were 4-0 down - Arsenal offered nothing in the final third.

But Arsene Wenger will likely be happy with the fight that his team showed in the closing stages of the match, as they were slowly allowed more time on the ball by Bayern.

Olivier Giroud was close to putting in another one of them performances. He was failing to have any meaningful impact on the game.

But one moment of brilliance saw him find the back of the net, proving that the Frenchman does still have something to offer.

Bayern's strikers were clinical as ever, with both Robert Lewandowski and Thomas Muller finding the net.

The pair of them both look exceptionally comfortable with the ball no matter how they receive it. At their feet or in the air, they constantly look a threat.

Lewandowski's goal was his seventh in seven Champions League appearances; Muller's strikes his sixth and seventh in six at home in the competition.

You look at Giroud and you know that he has the ability to be a top goalscorer, but he's just not got the same consistency levels as Bayern's duo.

Pep Guardiola led another Bayern masterclass

It's an opinion you have probably heard before: 'Pep Guardiola has only taken over great teams'.

It's true. He took a great core of Barcelona players when he first took the Nou Camp helm and he did the same when he was named as Bayern boss.

But the secret to Guardiola's success is not solely down to the calibre of players on the pitch (although it certainly helps). Their success is down to their immense discipline - which all stems from Guardiola.

He has drilled some of the most technically gifted players in the world to graft off the ball and they stopped Arsenal from having the freedom to play.

Very rarely did an Arsenal player get time to think on the ball. It's the high intensity approach off the ball that gives Bayern the upper hand in matches - just as Barcelona did under the Spaniard.

Guardiola is this generation's 'invincible'. Who knows what he could do in the Premier League.

Here are different opinions about the match on twitter:

@gooner8girl- I knew it was going to be bad, but 5? That's just embarrassing. I hope they take the frustration out on spurs

@joek_lib- Cech should endeavour to take pain relief drug after the match, even frog never jump reach this level

@dreamteamfc- foiled again, Arsene... foiled again

@willuvoh- The fear of losing 5-1..."I don't want to Arsene Weng"

@mplsharp- does wearing Arsenal pants and socks not count for anything anymore? It's as if my underwear has no bearing on my teams result. Awful

In other Champions League action:
Barcelona vs BATE 3-0
Roma vs Bayern Leverkusen 3-2
Olympiakos vs Dinamo 2-1
Chelsea vs Dynamo 2-1
Maccabi vs Porto 1-3
Gent vs Valencia 1-0
Lyon vs Zenit 0-2

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Same old, same old. English teams can't with Europe's elite! Deir style of football is jst inferior to deir counterparts from other leagues.

Bayern are a beast!!! I tell ya. If you play them you need to be alert, focused and passionate to even have a chance of beating them.

Arsenal did this at home. But away they lost focus. You could account it to the number of missing players who played the first leg. Or just pure intimidation because Bayern struck the fear of God into Arsenal.

But alas its not a shame to be beaten comprehensively by THE best club in the world. Arsenal need to lift their head up high and focus squarely on Totenham. That(premier league) should be their focus this season because we all know they cant win the UCL this season. Maybe next tym.

Lastly Arsenal NEED to investicate the root cause of their injury crisis. If its that the players are that physically prone. Then buy fitter and better players and leave those ones as back up. Because right now, Arsenal has zero back up.