Winning an English league title is no mean feat - plenty of capable teams have fallen short over the years. But successfully defending a crown is the ultimate challenge and what separates good sides from great ones.
Preston North End and Aston Villa in the 19th century, Liverpool, Sheffield Wednesday, Huddersfield and Arsenal in the inter-war years, Portsmouth in the late 1940s, Manchester United and Wolves in the 1950s, Liverpool in the 1970s and 1980s, Manchester United again in the 1990s and 2000s and Chelsea during the first Mourinho era.
All of these clubs won two, in some cases even three, consecutive Division One or Premier League titles. But curiously, in a modern era where many people have predicted several different clubs to begin periods of domination, it has been a full six years since anyone retained a crown. The last side to do so was Manchester United in 2008/09.
This season already looks set to be the seventh year in that growing trend after reigning champions Chelsea slipped 11 points behind Manchester City after the latest round of fixtures - a significant margin even at this very early stage of the campaign. To catch up, Jose Mourinho's team effectively require City to lose four games, while going unbeaten for the rest of the season themselves. Frankly, neither of which seems at all likely at this moment in time. That is also before considering the merits of any others involved in the title race.
The Champions League is notorious for being seemingly impossible to retain - not even the great Barcelona teams of 2009 and 2011 have done it - and now the Premier League is starting to follow suit.
When Manchester United couldn't retain their English crown in 2009/10, they only fell short by a single point. However, many onlookers pointed to the fact that Cristiano Ronaldo had been sold for a world record £80m and was not replaced.
As a result of huge spending sprees from Real Madrid and Manchester City, Sir Alex Ferguson highlighted the fact that there was no value in the transfer market. The great Scot was right, but his refusal to pay over the odds for a like-for-like talent meant his team had lost its lethal cutting edge. Instead, Gabriel Obertan, Michael Owen and Mame Biram Diouf arrived. Antonio Valencia also signed and enjoyed a promising first season, but was nothing like the talent of Ronaldo.
The theme of failing to replace talent, failing to strengthen and generally resting on laurels is something that has been a constant among title winners immediately relinquishing the trophy they fought so hard to win. It is a naivety that never ceases to be surprising given what it takes to be champions in the first place.
Manchester City have been terribly guilty of it in each of their two attempted title defences. On neither occasion (2012/13 and 2014/15) have the Sky Blues even come close to sealing back-to-back wins. In 2012/13, City were left in the metaphorical blocks by cross-town rivals United.
Having made concerted efforts to build a title winning team - bringing the likes of David Silva, Yaya Toure, Samir Nasri and Sergio Aguero to the club between 2010 and 2011 - they simply appeared to stop looking for the talent that would improve the squad, keep existing players on their toes and stay ahead of the competition. Of the six players who arrived at the Etihad Stadium that summer, only 37-year-old third choice goalkeeper Richard Wright remains.
City made exactly the same mistakes again two years later, while Manchester United had failed to 'top-up' their title winning team in the summer of 2013 when David Moyes arrived at Old Trafford as the new manager. It meant that the rest all promptly caught up.
It is the same naivety that has plagued Chelsea so far in 2015/16. The reigning champions have some of the world's best players in their strongest XI, but have allowed themselves to end up with a terribly weak squad where there is little to no competition for places.
Individuals like Branislav Ivanovic, Cesc Fabregas and Nemanja Matic have no one challenging them and it is showing in their performances.
The Premier League is highly regarded for its competitiveness, where anyone can genuinely beat anyone else on a given day - that kind of thing rarely happens in Spain and other European countries. There is no mystique or aura for defending English champions and it all means that when the naivety kicks in, as it has done in recent years, giants will always fall.
The Premier League title is not impossible to retain, of course it's not. But there isn't a divine right and no club will do it until they take the defence seriously enough.
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