Arsenal were "invincible" in 2003-04 Premier League. What the hell has happened since?

Lee Hendrie scored in the third minute to put Aston Villa in front, but it didn't matter. Arsenal would score and score again, and Arsenal would win. Because Arsenal always won.

Sure enough, Robert Pirès equalised from the penalty spot, and after Thierry Henry had given the home side the lead on the stroke of half-time, Pirès swept home his second goal to seal a routine 3-1 victory. "I don't think they would have panicked at even two goals down," beaten manager David O'Leary said. "They've got such belief."

It was Oct. 16, 2004, and Arsenal had just extended their record-breaking unbeaten run to 49 Premier League games. The defending champions were five points clear at the top of the table, having scored almost twice as many goals as any other team in the division, and with Villa duly dispatched, thoughts quickly turned to a midweek trip to Panathinaikos in the Champions League.

With the benefit of 15 years' hindsight, that mild, overcast October afternoon in north London turns out to have been the last time Arsenal looked unbeatable. Eight days later, their unbeaten run came to an end in a 2-0 defeat at Manchester United -- Ruud van Nistelrooy converted a penalty dubiously won by Wayne Rooney, who added a second goal in stoppage time -- and Arsenal have never been the same since.

-- Inside Messi10: The surreal, bizarre Barca star's circus show
-- Ogden: UEFA must get tough on Bulgaria after England game
-- Laurens: France haven't looked this bad in years

Arsène Wenger's side won only one of their next four league matches in the 2004-05 season -- a chaotic 5-4 success against Tottenham Hotspur at White Hart Lane -- and eventually finished 12 points behind José Mourinho's Chelsea in second place.

Since coming second to Chelsea at the end of the 2005 season, Arsenal have finished in the top two only once, in 2016, and even then it took a spectacular collapse from Tottenham for them to claim the step on the podium below Leicester City. Only the FA Cup, which Wenger won three times in his last five seasons, has provided any solace.

Over the course of Wenger's first 11 seasons (including the 1996-97 campaign, in which he took up his role in October), Arsenal conceded 369 league goals at an average of 33.5 per campaign. In his last 11 seasons, those figures jumped to 446 and 40.5 respectively.

Fifteen years on, Arsenal have yet to mount a serious title challenge, with the latter years of Wenger's tenure serving only to earn the club a reputation for psychological flimsiness and defensive fragility that remains very much intact despite his departure and Unai Emery's arrival. Once renowned for their robust back four, Arsenal are now the epitome of how not to defend. And it has been a long time since they looked anything close to invincible.

More damning than the statistics, though, were the defeats. The 8-2 at Old Trafford, the 5-1 at Anfield, the 6-0 at Stamford Bridge. The slapstick manner of defeat against Birmingham City in the 2011 League Cup final. The 10-2 evisceration by Bayern Munich on aggregate in the Champions League.

Piece by piece, brick by brick, Arsenal's empire quietly came apart.

"There was a burning desire not to concede"

Nigel Winterburn remembers a time when defensive discipline was at the heart of everything Arsenal stood for. A summer signing from Wimbledon in 1987, the former left-back vividly recalls the hours spent on the training pitches with the other members of Arsenal's famous back four -- Lee Dixon, Tony Adams and Steve Bould -- as manager George Graham led them through the drills that would turn them into the most formidable defensive unit in the English game.

With the four players aligned across the width of an otherwise empty pitch, Graham would stride around representing the ball, explaining how his defenders should react to each new position that he took up. In time, Arsenal's defenders learned to move as one, rolling up, down and across the pitch with the smooth, unthinking coordination of a shoal of fish.

"It became robotic," Winterburn told ESPN. "Once one reacted, the other three instantly knew what to do and where to go. That was the value of the work we did with George."

Graham's rigorous approach reached its peak in the 1990-91 season, when Arsenal won the title -- the second of his time in charge -- having conceded only 18 goals in 38 games. For the members of his defence, backed up by goalkeeper David Seaman, clean sheets were not so much a strategic objective as a raison d'être.

"We hated conceding goals," Winterburn said. "Even when we were 3-0 and 4-0 up. If we let a goal in late on and we ended up winning 3-1 or 4-1, we'd be absolutely livid with each other. There was a burning desire not to concede."

The contrast with the defensive apathy of the current team is striking.

When Wenger took over from Graham's successor, Bruce Rioch, in the autumn of 1996, he had the good sense to realise that the defence he had inherited would require little fine-tuning. Dixon and Winterburn, the two full-backs, were given more licence to push forwards in matches, but with Pat Rice, Wenger's assistant, adopting a light touch in his defensive training sessions, things otherwise remained much as they had been.

"To be quite honest with you, we didn't do a lot of defending [in training]," Winterburn said. "Pat took little bits, but I think he knew that we knew it anyway."

Boro Primorac, Wenger's right-hand man at Arsenal from 1997 to 2018, told ESPN in a rare interview that positional work remained a secondary consideration even after the team that would go on to be the "Invincibles" had been assembled.

"You'd repeat tactical work, but only 15 or 20 minutes -- not all day," Primorac said. "They were already ready. They were all big players. You can imagine, we had four or five world champions in this team. What can I teach a world champion? What can I teach Gilberto [Silva] about taking up positions on the pitch? He can teach me!"

As the players of the 1990s were phased out, Arsenal made a concerted effort to preserve the culture of defensive excellence they had helped to establish. Bould left in 1999, Winterburn in 2000, Adams and Dixon in 2002 (after Wenger's second Double), and Martin Keown in 2004, but as the players destined to replace them came in, the old guard ensured that the standards were kept while adapting to the increasingly attacking -- and successful -- football being developed by Wenger at the other end of the pitch.

Rather than collective movement, Arsenal's new defence was based on athleticism and stamina, with converted midfielders Lauren and Kolo Touré dropping back to play alongside Sol Campbell and Ashley Cole. Primorac describes them as "physical monsters." Danny Karbassiyoon, then a young left-back on the fringes of the squad, remembers being struck by the intensity of first-team training sessions.

"You can't just turn your intensity on and off like a tap. You're going to play the way you train," said Karbassiyoon, who now works for the club as a scout in North America.

"A great example was Keown, who was one of the hardest guys I ever had the pleasure of meeting or playing against. He'd be furious if he let in a goal or if his team lost. That embodied the attitude and the character of that entire team."

The "Invincibles" let in only 26 goals over the course of the 2003-04 campaign, figures bettered by only five title-winning teams in the Premier League era. But then came Old Trafford: the cynical targeting of José Antonio Reyes, Van Nistelrooy's studs down Cole's right shin, Rooney's dive, the flying pizza and the end of the unbeaten run. Wenger blasted referee Mike Riley's performance, claiming that Arsenal had been "robbed," and 15 years on, the manner of the defeat still rankles. "The referee was not correct," said Primorac, who now runs the academy at Hajduk Split. "The players were very, very upset."

The defeat at United also served to germinate the idea that the best way to unsettle Arsenal was to target them physically. As Arsenal faded over the seasons that followed, and as sturdy players such as Campbell, Lauren, Patrick Vieira and Gilberto moved on without being adequately replaced, Wenger's complaints about the lack of protection afforded to his players became a wearily recurrent refrain.

"If they couldn't play 'footballistically,' they would try to be aggressive," said Primorac, his choice of language reflecting the 21 years he spent at Wenger's side in the Arsenal dugout. "It's normal. They fought to win. It was a problem with the referees letting them kick us."

Whether at centre-back (Thomas Vermaelen, Sebastien Squillaci, Shkodran Mustafi) or in central midfield (Denilson, Kim Kallstrom, Granit Xhaka), the signings who should have made Arsenal stronger instead made them weaker, rendering the team ill-equipped to deal with the kind of rough treatment they received.

"Wenger misread what Barcelona were about"

Had the recent history of European football followed a different course, Wenger might have come to realise that the best way to inoculate his team against such rude physical treatment would have been to inject it with some hardiness of its own. Instead, the rise of Pep Guardiola's Barcelona convinced him that it was possible to play attractive, proactive football with small, technical players and still be successful. It was a formula that he grasped, albeit only partially.

"He misread what Barcelona were all about," former Arsenal midfielder Stewart Robson, who became one of Wenger's staunchest critics, told ESPN. "Whenever he was talking about Guardiola and Barcelona, he'd say, 'We're the closest thing to them. We play combination football, we've got small, skillful, technical players, and we can play around pressure.' What he didn't realise and acknowledge was that Barcelona were also the best team at closing the ball down. It wasn't just that they kept the ball, but they won it back so quickly.

"Arsenal didn't always do that, and you could tell from week to week, there was no defensive game plan."

By the time of Wenger's departure from the Emirates Stadium in May 2018, Arsenal's mounting frailties meant that even things the club's fans had come to depend on -- attractive football, consistent Champions League qualification, always finishing above Tottenham -- had slipped away.

"There's a lack of understanding"

When Emery took over from Wenger in the summer of 2018, he inherited a flaky squad with no clearly defined defensive principles, the influence of the old back four and the defenders from the "Invincibles" era having become more and more diluted as the long, slow second half of the Frenchman's reign wore on. Where once the Arsenal defence meant Adams' rugged tackling, Campbell's peerless heading ability or Cole's lung-busting surges down the left flank, it now stood for Xhaka failing to track his runner, Mustafi losing his man in the box or Petr Cech sombrely fishing the ball out of his own net.

At Sevilla, where he won three successive Europa League titles, Emery's teams were renowned for playing high-energy, relentlessly vertical football, but they were not an especially solid side defensively. His attempts to introduce a more dynamic style of play at Paris Saint-Germain were unsuccessful, and although PSG had few problems keeping clean sheets in Ligue 1, their frailties were brutally exposed in the famous 6-1 defeat by Barcelona in the Champions League in March 2017.

Emery hasn't yet had any discernible impact on Arsenal's defensive robustness. Arsenal conceded 51 goals in Wenger's final league season -- the club's worst defensive figures in 34 years -- and in Emery's first campaign at the helm, they shipped 51 goals again.

This season, things have been scarcely better. Emery's men are mid-table in terms of goals conceded (11), but their expected goals against figures (12.88) are the sixth-highest in the division, and only promoted sides Aston Villa and Norwich City have granted their opponents more shots on goal than the 136 Arsenal have allowed. Not that you need statistical evidence to tell you that all is not well in Arsenal's defensive third, where a porous midfield and calamity-prone centre-backs have combined to create a perfect storm.

Costly defensive mistakes have been a consistent feature of Arsenal football, be it Xhaka's kamikaze penalty-box challenge on Son Heung-min in the North London Derby, David Luiz's error-strewn display in the 3-1 defeat at Liverpool or Sokratis Papastathopoulos's catastrophic loose pass in the recent 2-2 draw at Watford.

"At the moment, it doesn't look any better than last year, which wasn't good enough. I think there's a lack of understanding between the back four, in terms of each individual player and the responsibilities in their roles," Winterburn said.

"And at times, the opposition play through the lines too easily. The more pressure that's put on the back four, the more mistakes they will make."

Can Emery turn things around?

Emery's thinking on defensive matters also seems muddled. He arrived at the Emirates promising aggressive, front-foot football, and there were signs of that in his first season, most memorably in December's 4-2 win over Spurs. But the intensity has steadily disappeared from Arsenal's play, and when they come up against supposedly inferior opponents, Emery can be curiously reactive, seemingly more inclined to focus on the strengths of the opposition than the strong points of his own side. On top of all that, his determination to see his team play out from the back can seem like more trouble than it's worth.

Pierre-Emerick Aubameyang complained to Canal+ after the draw at Watford that Arsenal were "literally giving goals to the opposition," but Emery has remained steadfast, declaring that building up play from the back needs to become part of the team's "identity."

Recent weeks, at least, have offered a faint promise of better days ahead. Arsenal have kept three clean sheets in their past four games across all competitions, and with Héctor Bellerín, Rob Holding and new signing Kieran Tierney all now free of injury, there is hope that a new defensive unit might emerge to put Emery's team on a surer footing.

"On the flip side, there's potentially three players who will be starting who haven't started [in the league] yet," Winterburn said. "Once you've had another seven or eight games, it might be the right time to have a look and see whether we've improved defensively or not. If we haven't, I think Unai Emery will have to answer a lot of questions."

Arsenal, the team who forgot how to lose, have become the team who forgot how to defend. Remember how to defend, and they might one day remember how to challenge for titles.