Football historians are divided on the root cause of the bitter rivalry between supporters of Southampton and Portsmouth. But such is the hostility that south-coast police on Saturday are being put on almost a war footing for FA Cup fourth-round tie between the two clubs.
Envy has certainly played a part. For most of the first half of the 20th century, Portsmouth were a prominent force in the English game, winning two Division One championships and the FA Cup while Southampton played in the lower leagues.
The tables turned in 1978 when The Saints were promoted to the top flight and looked down happily as a slump saw Pompey playing in the old Division Four a year later. But some reasons for the animosity do not just involve football.
There are a few surviving former merchant seamen in Southampton who remain convinced that naval ships from the Royal Dockyards in Portsmouth did not provide them with enough protection from German U-boats during the second world war.
Equally, there are pensioned-off stevedores in Portsmouth who still resent the dockers from Southampton who were moved along the coast to cross the picket lines in a 1950s dock strike. "Scum", an acronym standing for Southampton Company Union Men, is the derogatory term still used by Pompey fans to decry Saints players.
However, at lunchtime on Saturday, as the two sides meet at Southampton's St Mary's ground, it will not be the home players who will be the prime targets of such vitriol. Instead, it will be the three figures orchestrating their performance from the dugout.
If tension between the two clubs has mainly simmered below the surface for years - though it reappeared with a vengeance at last season's Premiership meeting at Portsmouth when street fighting led to 76 fans being convicted for violence - it is currently at boiling point.
The fires were fuelled in November when, two weeks after suddenly walking out on a glorious management spell at Portsmouth, Harry Redknapp accepted an offer to try to rescue Southampton from the Premier League relegation zone. He then compounded his felony in the eyes of Pompey fans by taking his assistant Jim Smith and head coach Kevin Bond with him.
With a sense of menace rather than mischief, one Portsmouth supporter reacted by posting Redknapp's mobile phone number on the internet, and the manager was so bombarded with threats and abuse that he was forced to close the number. It was not only an inconvenience to a man who spends many hours wheeling and dealing over players with a phone to his ear, but also an eye-opener too.
Although Redknapp has spent this week trying to avoid media interest about his part in what has become a south-coast soap opera, the much travelled Smith said the hostility had been a severe shock to their systems. Although Rangers against Celtic or Arsenal-Tottenham might be said to epitomise derby rivalry in terms of big-club tribal warfare with a religious or ethnic element, Smith said the only similar intensity of ill-feeling between Portsmouth and Southampton that he knew of was that involving the clubs of the Lancashire cotton towns of Burnley and Blackburn.
"That's the only one I have seen as serious and aggressive as this one is," said Smith. "I got a real shock from the reaction of the Portsmouth people. Harry just wants to manage and be involved in football and I think he, too, was genuinely shocked and hurt by certain fans' response. It wasn't all like that - he had nice letters as well, but nobody wants to hear about them do they?"
What the public has heard about is the spat between Pompey chairman Milan Mandaric and Southampton counterpart Rupert Lowe over the handling of Redknapp's change of employer. They managed to agree cordially the purchase by Redknapp from Portsmouth of Scottish midfielder Nigel Quashie for £2.1m this month, to the further chagrin of Portsmouth fans, but the chairmen have, nevertheless, continued sniping at each other, further raising the temperature for today's game.
Almost three months on from Redknapp's move to St Mary's, there appear to be signs of another shift in the fortunes of the two clubs. A convincing victory over Liverpool last weekend, Redknapp's first in eight league attempts at Southampton, has raised hopes of avoiding relegation and boosted confidence before today's game.
In contrast, although Redknapp's successor Velimir Zajec had a flying start with Portsmouth, giving Arsenal a tough time before losing to a deflected goal, and then winning points at Newcastle and Liverpool, the new year has not begun well. Three successive league defeats have seen the club start to slip from mid-table security in the Premier League, and the departures of Quashie and Amdy Faye (to Newcastle) have left a big hole in Zajec's midfield.
On Friday the Croatian, who was formerly director of football at Panathinaikos in Athens, signed goalkeeper Konstantinos Chalkias and midfielder Giannis Skopelitis from his former club, and both could make their debuts in Saturday's game.
After 90 minutes of hopefully only verbal warfare on the terraces, they may both wonder what they have let themselves in for.