The ultra groups of Spain's premier division teams have their own names. Athletic Bilbao's hardcore fans are called Herri Norte Taldea (HNT); Real Madrid's are known as the Ultras Sur. Neither group likes the other.
There's a song that HNT sing whenever Athletic play Real Madrid (a melody to the tune of "When the Saints Go Marchin' In"): "We don't like violence in football grounds/We don't like violence in San Mames/We have to kill, we have to kill/We have to kill an Ultra Sur/We have to kill an Ultra Sur/We have to kill an Ultra Sur."
The hatred between the two fan groups comes easily. Politically, HNT have left-wing Basque independence tendencies, while the Ultras Sur are a cadre of right-wing Spanish nationalists. In January 1997, Jose Luis Ochaita, the Ultras Sur leader at the time, was stopped by security trying to enter the Santiago Bernabeu before a match against Athletic with a metal door handle and three screw guns, as detailed in Carles Vinas Gracia's book El Mundo Ultra.
In the same fixture a year later, some HNT fans weren't allowed inside Real's home with 31 baseball bats. Although it has rarely spilled over into violence, there has always been tension in the terraces when the two sides meet, a flavour of which Real Madrid can expect when they visit Athletic's stadium, San Mames, on Saturday in La Liga.
"Since the late 1970s and into the 1980s, the welcome for Real Madrid has been very, very hostile," says Professor Angel Iturriaga, author of Dictionary of Athletic Club Players. "The atmosphere has always been very heated when Real Madrid arrive at San Mames, especially after General Franco's death in 1975, but 'the blood never came to the river,' as we say in Spain.
"There has never been a big confrontation or bad violence between the fans. It has always been more like verbal violence, a war of words. It doesn't get physical or out of hand. Maybe in some isolated moments there has been some aggression, towards a bus of fans, for example, but it usually never goes beyond being the threat of violence.
"The ambience is a lot more charged politically in Bilbao compared to the return fixture in Madrid. In the terraces at San Mames, there are always banners critical of Real Madrid as the representative of centralised power in Spain. Until recently, you used to see a lot of Athletic fans with banners supporting the release of ETA [a Basque armed separatist group] prisoners, or the transfer of them closer to the Basque Country.
"You will also see banners seeking independence for the Basque Country from Spain. Athletic Bilbao, the club, has always been linked very closely with PNV, the Basque Nationalist Party, and its more radical fans are linked to the Basque left-wing independence movement."
The two clubs have an ancient rivalry with claims to being the oldest, sustained matchup in the history of Spanish football. It is sometimes referred to as "El Viejo Clasico" (The Old Classic). Both clubs were dominant forces in Spain in the first half of the 20th century, winning La Liga seven times between them after its formation in 1929 (Athletic captured five of those titles).
Until December 2011 (when it was overtaken by Barca-Real Madrid), it was the most-played fixture in Spanish club football. The change reflected the ascension of Barcelona as Real Madrid's key rivals since Johan Cruyff's Dream Team won four league titles on the bounce in the early 1990s. When Cruyff took over as Barca manager in 1988, it was just as Athletic Bilbao had begun to fade domestically, failing to win a league title since 1984. The three clubs are the only ones to have retained premier-division status every season since La Liga began.
Athletic vs. Real Madrid shares the same political dynamics as the Barca-Madrid rivalry: both Athletic (Basque Country) and Barcelona (Catalonia) are flag-bearers for their regions against the might of central Spain, Real Madrid. The Basque Country, with Bilbao as its capital, and Catalonia are two of the main engines of the Spanish economy; both regions have their own languages, Euskara and Catalan.
In 2011, ETA declared a ceasefire. The Basque people instinctively know their separatist history. Writer Phil Ball, author of Spanish football rivalries book Morbo who has lived in the Basque Country since 1991, believes it's ingrained. The political antecedents of the rivalry are part of the muscle memory for fans.
"Spanish people are very atavistic," he says. "They know exactly what went on. They're pretty smart in that sense. They might not be able to tell you dates and so on but they know the basic cultural norms. They all conform to them. The cultural norm in Athletic is to hate Real Madrid. Why? Because of the Franco dictatorship and its history.
"The Basques have their view of Spain, and Madrid has a different view of Spain. If you took a cross section of Athletic fans and asked them who was their most despised enemy, they would say Real Madrid. That Madridistas still represent this bulls--t idea of 'Madrid casta'—that they are a caste that has sustained Spain."
Ball believes the drivers for the enmity have altered, however, since the era of Florentino Perez's presidency and the galactico project he began at Real Madrid in 2000. Real Madrid has become a global marketing phenomenon. The team is now a multinational group stocked with some of the best-paid players in football, such as Thibaut Courtois (Belgium), Marcelo (Brazil), Toni Kroos (Germany) and Gareth Bale (Wales). The club is no longer a bastion of Spanish identity.
"There has been a bit of a change at the Bernabeu in the last 20 years," says Ball. "Real Madrid has become this sort of chulo [cocky], moneymaking machine. They don't really stand for anything anymore. There's recognition that Real Madrid is now just a huge multinational, global thing. They're not trying to be super Spanish anymore, 'looking down on us Basques.' They're disliked for a wider range of reasons than before when it was 'Franco's team.'
"How do you project your political hatred on someone like Cristiano Ronaldo, who came up to Bilbao scoring goals for nine seasons? If you hated him, it's because of who he is—an arrogant goalscorer. It's not like Juanito [Real Madrid, 1977-1987] coming up. They couldn't stand him. They chanted some horrible things when he died in a car crash [in 1992].
"Fernando Hierro [Real Madrid, 1989-2003] was a hate figure. Or [current director] Emilio Butragueno [Real Madrid, 1984-1995], for all his little-boy looks, is everything that your left-wing Basque despises. What is there left to hate now? There's only a remnant of that hatred for an older generation. It's shifted. I'm not saying they don't dislike them, but it's for different reasons.
"There is a misconception that [the fans'] political viewpoints are really driving the rivalry. I don't think they are anymore. Athletic's outlook and philosophy hasn't changed, but the nature of Real Madrid has shifted. The hatred is no longer purely political. It might be the perception but it's only partly political."
Athletic are revered the world over for their policy of using only players who are born in the Basque region or who have been schooled in Basque football academies. Perhaps the most notable modern example is Aymeric Laporte, who joined the club's youth academy as a teenager before playing for the first team and eventually being sold to Manchester City in January 2018 for a reported fee of €65 million. Real Madrid fans look on the Basque-only policy ambivalently, though.
"In a part of Madrid there is sympathy for Athletic Bilbao and its philosophy," says Iturriaga. "It's a sympathy that you can see in almost all of Spanish football—for what the philosophy means, not to sign foreigners, to play only with home-grown players. All this makes Athletic very beloved by less radical Real Madrid fans that are more into their football. There is a bond, an appreciation for that idea.
"It's possibly a minority of their fans who see them with this perspective, though—in a positive way. The majority see them actually as a Basque rival—far away from what Spain represents. They don't understand their vasquismo (Basque identity). It's beyond their parameters."
Athletic will be looking to defeat Real Madrid on Saturday at San Mames, something that has been beyond the club since early 2015, when Aritz Aduriz scored in a 1-0 victory. It will be a fiery atmosphere in San Mames, even if the blood doesn't flow to the river.
Follow Richard on Twitter: @Richard_Fitz