The takeover of North London by German fans and the subsequent chaos at the Arsenal vs. Cologne Europa League match on Thursday night throws up wider issues surrounding the behaviour and perception of football supporters in England.
Reaction to the events at the Emirates has been mixed. Some commentators have interpreted the chaos around and inside the stadium as a dangerous and a throwback to the days when violence stalked the game. Others have lauded the passion of the German supporters and used the incident to highlight the sterile nature of Premier League grounds.
Public safety should be paramount, but the hysteria about the breakdown in segregation has its roots in a different era.
Many Arsenal fans were shocked to find Cologne supporters in sections that are usually exclusively populated by home fans. This, as much as anything, created fear and panic in the stands of the Emirates.
In England, supporters have become conditioned to watching matches exclusively alongside fellow fans of their club. Away ends are sectioned off, often with rows of empty seats and a line of stewards to ensure contact between the two disparate groups never takes place.
But it was not always this way. Segregation in English grounds was less formal until the 1960s. There were always areas where home and away fans gravitated towards, but supporters were generally free to enter any area of the stadium. Two matches at Upton Park in 1967 changed this.
Manchester United went to East London in May on their way to winning the title. United fans swamped Upton Park and were in every part of the ground; West Ham United, as a club, were unprepared for the influx of away supporters, as were the police. The home fans were equally surprised and there was serious disorder on the terraces and in the stands.
The fixture computer matched the same two teams at the same venue in September. West Ham supporters were looking for revenge and it was another nasty afternoon in East London.
After these high-profile incidents, the policing of football matches in England began to change. The authorities went to great lengths to keep different sets of supporters apart and travelling fans were kept away from their hosts inside the stadium by being placed in separate pens behind fences.
The policies did little to stop the growth and spread of hooliganism. In some ways it made the situation worse. An antagonistic "us-and-them" attitude grew; provocative behaviour and chanting on both sides of the divide went unchallenged. Ritualised abuse became part of the game, and the strict division of stadiums created flashpoints where confrontation across the barriers was a normal event.
Thankfully, the days of regular violence in football have long gone, but some of the effects and traditions of those days remain.
It is still considered unacceptable by many supporters to see opposition fans sitting in the home section. If any "outsiders" are spotted there is often a clamour for stewards to eject them from the ground, even if they are no threat at all to those sitting around them.
Occasionally it becomes ludicrous: there have been many incidents where families and children have been led out the stadium for having the temerity to celebrate an away team's goal while seated among the home supporters. The response of Arsenal fans after finding German visitors in the same section was conditioned by this attitude.
Fear and anger towards opposition supporters still has a deep-seated hold on English football. Other sports do not have such strict segregation. In Rugby League, a sport with a similar demographic, supporters mingle to a much greater extent, and mixed areas are more common.
Many of the attitudes to policing football and policies affecting supporters are stuck in a different age. Football fans can be prosecuted for consuming alcohol in sight of a pitch; in other sports fans happily sip beer while watching the action. This is the result of anachronistic legislation enacted more than 30 years ago.
The creation of "bubble matches" is even more insidious. These are games where away supporters can only attend the match if they travel from designated pickup points on heavily policed transport. In one surreal situation, a Hull City fan living in Huddersfield was forced to travel to Hull to pick up a coach when his team played Huddersfield Town.
The trouble at the Emirates needs to be investigated, and the failures of security and policing should be addressed. Yet it is also time for a wider discussion of how supporters are treated by the authorities -- and how they treat their rival fans.
There are different methods of policing public events. One extreme is to assume that people are enjoying themselves and approach it as a party. The other is to prepare for a riot. Too often, football is still seen as being on the troublesome side of the spectrum.
The incidents at the Emirates should be the starting point for a wider debate about fandom. Accepting other people's right to support and celebrate another team without seeing that behavior as offensive and threatening would be a good start.