It was in an FA Cup semi-final that the new Manchester City traversed the Red Rubicon of overcoming Manchester United in a crucial fixture.
The psychological value was not in what the Wembley win inflicted their rivals, but in what it brought.
England's oldest trophy was there for the taking and Roberto Mancini was able to fulfil his pre-season promise to deliver at least one trophy and with it strengthen his position at the club.
With one memorable victory over United, the Italian was in a position to cajole the club's Abu Dhabi owners into more domineering purchases.
Sergio Aguero was rapidly secured as insurance against Carlos Tevez's temper tantrums, and Samir Nasri was lured away from Arsenal.
City's squad grew stronger and Mancini reinforced himself as its leader.
Sir Alex Ferguson now enters this repeat encounter in a testing position.
Injuries have emphasised the inexperience of sections of his squad as he repeatedly cools supporter demands for January reinforcement; his pre-match analysis of City's challenge is "that we may as well get used to it because they are not going to go away".
Mancini intends to press home his advantage. Though his employers want him to work with City's already considerable resources, the Italian is once again pushing them "to do everything we can to win", aware that another derby triumph can only aid his case.
This is no mere cup tie, it's a battle in the war of Manchester.