Some might explain the animosity between Jose Mourinho and Josep Guardiola as the result of a clash of footballing styles. From this point of view, the latter's almost obsessive predilection for ball possession couldn't possibly get along with the former's physical approach to the beautiful game. It's a question of creeds.
Others believe that it's just the almost predictable outcome of two determined, addicted-to-winning coaches who managed the two biggest Spanish clubs at the same time during two heated seasons.
Only one of them could win; the huge stakes, fuelled by the amount of noise generated by media outlets in Madrid and Barcelona, made them collide in spectacular fashion. From this perspective, fate would have put them in such circumstances that they were almost destined to fight.
Both explanations ring true, depending on the part of those two seasons that one examines more closely, but there is indeed something else to it. At the end of the day, plenty of coaches with huge differences get on famously well, while a number of Real Madrid and Barcelona coaches and players are extremely close friends despite the rivalry.
History may help to clarify the reasons behind their mutual contempt.
When Mourinho arrived in Madrid in May 2010, he didn't like what he saw. Despite having broken the club's point record in La Liga under Manuel Pellegrini, the team had lost the tournament to Guardiola's Barcelona; the Madridistas had also been eliminated from the Champions League in the last-16 round for the sixth consecutive season, and had embarrassed themselves in the Copa del Rey with a 4-0 defeat to Third Division Alcorcon.
Mourinho set about the task of removing Barcelona and Guardiola from their throne. However, a streak of 19 consecutive undefeated matches since he took over the team came to a screeching halt at the Camp Nou. Real Madrid's 5-0 defeat in late November 2010 would be later hailed by Barcelona midfielder Xavi Hernandez as the best game he'd ever played in.
It also had a remarkable impact on Mourinho. Just as he'd done in previous phases of his career, the Portuguese coach resorted to playing games on and off the pitch.
His mind games started with a publicised list of 13 refereeing mistakes, allegedly committed by a referee in a match against Sevilla in late December. He continued by publicly stating that he was tired of fighting alone against the refs and the system, which favoured Barcelona, with no help from his own club.
Obviously, his constant tirades generated a deafening amount of noise among fans, media and even players. For the most part, Guardiola kept quiet, true to his reluctance to give interviews and speaking only when he was obligated to do so by UEFA or the Spanish FA.
The 'morbo' reached extraordinary proportions during a period of 18 days in 2011 where both teams played four 'clasicos' across domestic and European competitions. The boiling point arrived in the days leading up to the first leg of their meeting in the semi-final of the Champions League in April 2011.
Two days before the match, Mourinho stated that there only two groups of coaches: "those who never speak about refs and a larger one, the one I belong to, which criticises refs when they make relevant mistakes. With Pep's latest statements we enter a new era, with a third group with only one member, who criticises refs when they are right. I'd never seen this."
The Portuguese coach was referring to Guardiola's press conference the previous weekend, during which he'd said that "a linesman with excellent eyesight has correctly ruled out a goal by Pedro by two centimetres."
After several other previous mocking statements from Mourinho, it's hard to see why Guardiola exploded at this particular one, but he did.
In his own presser before the semi-final, he truly let fly: "Tomorrow we are going to play a game... Off the pitch he’s already won, he’s been winning all year ... I’m happy to award him his personal ‘Champions League’… off the pitch. In this particular press room he’s the (expletive) boss, the big (expletive) chief."
Not one to stay quiet, after Real Madrid's subsequent 2-0 defeat with a couple of controversial refereeing decisions, Mourinho delivered his most delirious rant: his ad-lib speech after the match included lines such as "One day I'd like Josep Guardiola to win this competition properly," in reference to Barcelona's win over Chelsea in the 2009 semi-final.
From that point on, all bridges were burned. Their relationship never recovered.
Years later, they go to great lengths to remain civil in public, the memories of their time in Spain have proven hard to erase. But what could at some point have been explained as a clash of cultures or a result of fate bringing them together, it became something personal during those days in April 2011. It will be hard for them to hide that during their debut on their new stage in Manchester.