United suffered their worst home loss since 1955, when they lost 5-0 against City.
Saturday marks exactly 10 year since Manchester's football landscape was changed. The so-called "noisy neighbors" began to party on the front lawns of their rivals.
Manchester City's 6-1 win in the derby could have been regarded as an isolated event. However, it is best remembered for Mario Balotelli’s "Why Always Me?" celebration. After putting the visitors ahead, he wore a T-shirt.
It has been a lasting symbol for both clubs. Not just because City's win at Old Trafford would ultimately see them win that season's title on goal differential, ahead of Manchester United.
This day marked the beginning of a power shift in the city. The Blues began their land-grab for Red territory and did so in stunning style.
Since October 2011, City has won six League Cups, one FA Cup and one League Cup. United, however have won every domestic cup competition once.
City has delivered emphatically on what was then seen as a statement. Their stature and trophy haul have soared in each season since, while only sporadic United successes have followed.
United's dominance was ended in this area, in the Premier League. As their patchy form and waiting for their first piece of silverware under Ole Gunnar Solskjaer continue to show, their old aura may yet return.
City was the one playing catch-up
United boss Sir Alex Ferguson gave City the "noisy neighbor" label in the months following their Abu Dhabi takeover in 2008. However, three years later, City was still used to being ignored by the team down the road.
Roberto Mancini's team had something to celebrate when they beat United at Wembley, in an FA Cup semi final in April 2011. However, the ambitions of the new Blues regime, and a string of costly signings, had not changed the order of play in Manchester.
Sir Alex Ferguson's comment about City's "noisy neighbor" was prompted by their billboard campaign at Manchester city centre. They had signed ex-United striker Carlos Tevez back in July 2009. After United had won the Old Trafford Derby 4-3 in September that year, he made the analogy and said: "Sometimes you just have to live with a noisy neighbor." They will always be loud and you can't do anything about it. It is okay to just get on with your day, turn up the volume on your TV and go about your business.
United had won five of six league matches with Sheikh Mansour's City and had also won one. They were also on an 18 month unbeaten run at home, a venue City had only won once in 32 visits since Denis Law’s famous backheel in 1974, when his old side was relegated from the second tier.
City were seen as underdogs on the road to Old Trafford. This was the place where Premier League champions were based.
They were never seen in that light again after what happened next.
Balotelli kicks things off with a bang
Balotelli is the person you would most like to live a quiet and peaceful life with.
A firework had been set off in his bathroom, setting it ablaze.
The headline-grabbing maverick Italian striker was not only a familiar face, but he had also planned his Old Trafford celebrations earlier in the week, and printed his T shirt days before the pyrotechnic antics. But it would be a fitting way for an explosive day to start with a bang.
There would be more rockets. City led only 1-0 at halftime, but they took control when Jonny Evans was sent off to bring down Balotelli in the second period. They were now 3-0 up with 21 minutes remaining.
United had come back from 2-0 down to beat City at the Community Shield in August. However, their track record of rescuing seemingly abandoned causes proved to be their undoing. Darren Fletcher reduced United's deficit to 3-1 with nine more minutes to go.
As they tried to find more goals, the hosts left large gaps and were brutally punished by David Silva (89 mins) and Edin Dzeko (91 & 93).
Dzeko's second victory was his best. He ran on to a beautiful volleyed pass by Silva, before slipping home. After celebrating, he gave a six-finger salute in front a United banner that read 'MUFC-Not Arrogant, Just better', which made for an ironic backdrop to a surreal day.
A terrible defeat - Sir Alex Ferguson
Ferguson, a weeping man, told Sky Sports cameras that "we just kept attacking" right after the last whistle. "At 4-1 we should have said "we've had our day", but we kept attacking with full-backs forward, leaving them three against 2. It was insane, crazy football.
It was a terrible day. It's our worst ever day. We will return, given the club's nature, but what worries me most is the goals for and against now. It was 10 goals that made the difference today.
"That was a huge blow for us, because you never know when goal difference might count. It is usually in our favor, but at the moment it's not.
Ferguson was correct, but the damage was done. This scoreline would be decisive, despite all the drama that was to follow on the last day of campaign.
City was no longer afraid of Old Trafford - it has been a happy hunting ground ever since.
The 2011-12 Premier League title was decided on goal difference for the first time. City's finished eight points ahead of United's. City would have won 6-1 at Old Trafford if it had won 2-1. This would have given both teams identical records, and forced a play-off.
"This city is ours"
City fans sing about the game often, and verses about each goal they scored between 3-1 and 4-1 were greeted with a large exodus from United fans.
It also includes the claim, which has been slightly modified here with a rude replacement - "you lucky devils it should have 10".
This part of the song is open to debate. However, it is less doubtful than the last assertion that "this is our city" in the song.
"This is our City" - Since winning 6-1 at Old Trafford a decade ago the Blues have held the lead over United in terms trophies, league positions, and head-to-head matches
Both clubs have had spells in the past where Manchester was clearly Manchester's dominant force. City, however, were the early frontrunners based on their finishing league positions.
The Blues won 27 of the 37 seasons between 1897 and 1938 and also managed to do so in six of the 10 years of the 1970s.
United performed better from 1980 to 2011 than any other occasion, except for 1991, which was the year before the Premier League era began.
This was all overturned by this humiliation. United would win the Premier League again in 2013, but that is their last title. Their fans had to learn to look up at the table and not down to see City, who have finished ahead of them nine times in ten since their 6-1 win. This includes each of the eight previous seasons.
Overall City is only ahead of the next head-to-heads 11-10. However, United now dominates Old Trafford, where they have won five of their nine league games since 2011, losing only once.
City has won seven of the 12 matches in all competitions with only three losses. They will be back on 6 Nov as favorites to win again.
They will need something extraordinary to surpass their decade-old deeds. United's ongoing challenge is to find a way to muffle them.
