When Benfica faced Bayern Munich in the UEFA Champions League in September, a loud, almost-deafening whistle from around 60,000 fans was heard when the substitute board went up in the 79th minute.

James Rodriguez's No. 11 flashed up, and as Bayern's Colombian maestro slowly walked off the pitch, the boos became even more ferocious.

James decided to respond to them by holding up five fingers to the Lisbon crowd. That led to Benfica's centre-back and captain, Jardel, rushing towards him. He had to be stopped to prevent a head-to-head confrontation, while Benfica coach Rui Vitoria remonstrated to Bayern counterpart Niko Kovac about the incident.

Later that night, in the mixed zone, Rodriguez explained the gesture had been a reference to Porto's 5-0 thrashing over Benfica in 2010. "I don't think it's a big deal," he said. "Their fans didn't like it? Too bad. I'm a 'Portista' [Porto fan] and they hate Porto here. I didn't enjoy being booed either, but that's normal."

James had a hugely successful spell with Porto, winning eight trophies in just three seasons with the club between 2010 and 2013.

He probably learned over that time the Benfica-Porto rivalry goes way beyond football. It's a match that divides an entire nation like few others do.

It's the more laidback, powerful and wealthier south facing the industrial and hardworking north. The capital Lisbon against the second-largest city in the country. The "alfacinhas" (little lettuces) versus the "tripeiros" (tripe-eaters). A tale of two regions that compete against each other in seemingly every area of public life.

"In Porto, they work. In Lisbon, they spend the money," goes a famous saying in Portugal.

Once again, Benfica and Porto will bring these differences and disputes to Estadio da Luz in Lisbon on Sunday.

When the two Portuguese giants meet, it's never just a game.

Every tackle, drop of sweat and second ball is fought over. The bitterness between them has not subsided over time.

Stories about police searches, hacked emails, espionage, bribe accusations and even witch doctors have added fuel to the flames. The atmosphere around "O Classico" has arguably never been hotter.

In the past few weeks, Benfica have been formally accused of corruption by Portugal's state prosecutor. The case began after Porto's communication director, Francisco J. Marques, revealed a series of internal emails from their Lisbon rivals. Benfica have said that an investigation will confirm that Porto paid a hacker based in Budapest, Hungary, to steal their emails. Marques has denied the allegation.

"Of course, they [Benfica] find it difficult to believe that FC Porto did not pay a penny for the emails. It's in their blood, after all, they are the only SAD [a public limited company operating in the football industry] that has ever been accused of corruption," he reacted on Twitter.

Javier Martin Del Barrio of Spanish newspaper El Pais wrote in March that "probably there isn't a country in the world with more peaceful athletes and more violent leaders. That's Portugal."

In a war environment like this, it takes a lot of guts for someone to move from one side to another. Still, a few players have been brave enough to do it, and Bleacher Report spoke to two of them.

A former international with 31 appearances, Rui Aguas was one of Portugal's greatest strikers and joint-top scorer in the 1987-88 European Cup, when Benfica reached the final. His father, Jose Aguas, was also a club legend, so no one expected him to sign for Porto.

He did it anyway—and he regrets the move to this day.

In the late '80s, the move made financial sense. Despite being a fan favourite, Aguas was frustrated by his salary at Benfica, and when Porto offered him 11 times what he was making in the capital, it was enough to lure him north. Money talked, but fans have long memories.

"After all this time, 30 years, some people still come to me and say they were disappointed. ... That's how big the rivalry between Benfica and Porto is," Aguas, who now works as the Cape Verde national team coach, told B/R.

"It's not only about sports. It's also political, particularly in Porto. I played for them, got to know how they prepare for a Classico and it's different. For them, when they walk up the stairs and enter the pitch to face Benfica, it's the Porto cause at stake, the team, the city, the region. It's not just a game, it's much more than that.

"They've got the impression that Lisbon is overprotected, has privileges, much of that for being the capital. They may be exaggerating. Or perhaps they are right. In a way, not literally, it's them against the rest of the world."

As mad as it might sound, despite being called a traitor, Aguas still returned to Benfica right after his spell with Porto and had one of the best seasons of his career. It was still not enough, however, to win the fans back.

Joao Manuel Pinto, a solid centre-back, won four consecutive Primeira Liga titles with Porto in the '90s before making the switch south to Benfica in 2001.

He is more philosophical about crossing Portuguese football's great divide.  

"No one wants to lose a footballer to the rival team, especially if you rate him. When I moved [to Benfica], the Porto fans obviously didn't like it, but I can't complain. Footballers have a very short career; normally you don't get more than 12 years in elite competition and need to make the best out of it [financially]," Pinto says.

"In other leagues, it doesn't seem to be a big deal [to move between rivals]. But it remains an issue in Portugal. I suppose we are unique in this way."

It's safe to say that Portugal's international results in the past were not always helped by the hostile atmosphere between those who represented the red of Benfica and the blue and white of Porto.

"If you compare to the current national side, the [atmosphere in the] dressing room is very different because of a fundamental detail. In the past, we were all playing for Portuguese sides and dealing with this rivalry on a daily basis. Nowadays, most of them play abroad. It's not the same," Aguas argues.

"Inside the national team bus, Benfica's players used to sit in the front, Porto's in the back and footballers from Sporting CP and other teams in the middle. We didn't share the same table for meals, either. Despite all this, we respected each other and got along during the 1986 World Cup. But obviously it's difficult for a team to succeed in such an environment; friendship is important."

Sociologist, writer and football pundit Joao Nuno Coelho said it has not always been like this in Portugal.

"Everything changed in the '30s, when the two teams started to take part in official competitions—their relationship suddenly got complicated, with both of them reaching finals and seeing insults, pitch invasions and game-abandonment incidents arising," Coelho says.

"It's worth mentioning Porto established a tradition of securing the titles of the debut editions of Portugal's biggest tournaments. It was a leverage they had in any discussion with Lisbon powerhouses.

"In the '40s, '50s and '60s, Porto could not maintain their position and contest Lisbon's influence—including Benfica, Sporting and also Belenenses. Throughout that time, Porto won the league only twice, in 1956 and 1959, and was called Andebol Clube do Porto by their rivals [due to their exclusive success in handball]."

Porto's resurgence in football coincided with the arrival of Jorge Nuno Pinto da Costa and Jose Maria Pedroto as director and coach, respectively, in the late '70s.

Beating Lisbon became the driving motivation for the club.

Pedroto is famous for saying "a trophy won by Porto is worth two times or more than those from Lisbon teams." He is responsible for ending the so-called "complexo da ponte" (bridge complex)—a phrase that implied Porto started losing games the moment they crossed the Douro River to play in Lisbon.

"Pinto da Costa and Pedroto were not afraid to set up a confrontation strategy against the centralist power of Lisbon, which, according to them, benefited the local sides, mainly Benfica. That naturally led to the increasing animosity between Porto and Benfica," Coelho explains.

"But it paid off for Pinto da Costa, as he became Porto's chairman in 1982 and saw the club establishing a progressive dominance in Portuguese football. From five league titles in 43 seasons between 1934 and 1977, to 23 trophies in 40 seasons from 1978 to 2018. They became the most successful team in Portugal after the April 25 Revolution [in 1974, when the democracy was restored in the country]."

Despite being formally accused of corruption and then absolved, Pinto da Costa remains Porto's president after three decades.

A controversial figure, the 80-year-old has overseen the most titles in world football as a club president and, after all this time, he has not softened a bit in his battle against Lisbon.

In 2013, he listened to Antonio Mexia, an executive for EDP (one of Europe's main electricity operators), as he argued that a national title for Benfica would help increase the GDP of the country. He was infuriated and quickly replied that Porto would be looking for a new energy supplier.

Porto then went on to disappoint Mexia and Benfica by winning the Primeira Liga trophy in 2012-13, with a crucial winning goal in the last minute of the Classico on the penultimate weekend of the season.

Portugal's economy didn't seem to suffer, and now the country has actually been heralded as one of Europe's economic success stories.

For better or worse, there are no limits when Benfica and Porto meet—not even the economy is left out of the discussion.

The latest edition takes place on Sunday, and as always, it will be worth following—on and off the pitch.