[18][19][20] They also place great importance in developing local players through their cantera (youth systems),[21][22][23][24] and were among the last major clubs to adopt a commercial sponsor logo on their jerseys (Athletic doing so in 2008 and Barcelona three years later).

In that period, the two clubs introduced measures which led to them becoming important symbols of their respective homelands,[35][36] Athletic implementing a policy of using only local players in response to being criticised by opponents for selecting too many foreigners,[37][38] and Barça adopting Catalan as their official language.

The teams did not meet again prior to the establishment in 1929 of a national professional league championship, La Liga, which would see them face off regularly and become rivals for the title:[6] Barcelona won the first edition but failed to make much impact thereafter, suffering the humiliation of a 12–1 defeat at the hands of Athletic Bilbao in 1931 (Bata scoring seven in what still ranks as the biggest win in the history of the competition)[3][39][40][33] and also losing the 1932 Copa del Rey Final to the Lions.

In broad terms, Catalonia and most parts of the Basque Country fought on the losing Republican side in the conflict, and the victorious Nationalist regime soon introduced measures against regional languages and cultures.

[46] Real Madrid had moved to a larger ground a few years earlier, and they too signed talent from Eastern Europe and South America as well as from around Spain which propelled them to further domestic and continental success, with the positive international exposure pleasing the regime.

The two clubs had already played both league fixtures that season, the Catalans winning 3–1 in Bilbao and the Basques claiming a 2–0 victory at the Camp Nou ten days before their UEFA Cup tie.

[56] The tie was played in an unusually friendly atmosphere due to the similar regional identity of the two clubs, whose supporters were excited by the prospect of a brighter future for their respective territories after the death of dictator General Franco and the weakening of his regime.

[11] They had paid a world record transfer fee to bring Argentina star Diego Maradona to the club, followed in 1983 by his compatriot and mentor César Luis Menotti as head coach.

[9] However, besides Real Sociedad (who were weakened when midfielder Perico Alonso moved to Barça after their second title), it was also a strong period for Athletic Bilbao, who developed a group of talented but highly aggressive and physical players under coach Javier Clemente.

[11] It was clear that the following season could be a close affair between the three clubs, with the added elements of a clash in style and personality between Menotti and Clemente,[9][62] who were not afraid of expressing their opinions to the press,[11][9][68][69] and the pervading feeling in Barcelona that their 1982 title bid had been ruined by a serious knee injury to influential midfielder Bernd Schuster sustained in a challenge from Athletic defender Andoni Goikoetxea.

[70] Goikoetxea, from then on frequently referred to as the Butcher of Bilbao after the incident became widely publicised,[70] was given an 18-match ban,[9] soon reduced to nine then six,[11][68] meaning he returned in November, and his importance to the team was underlined by the fact those were the only league matches he missed all season.

[73] Neither player took part in either leg of the autumn 1983 Supercopa de España between the sides, won 3–2 by Barcelona on aggregate,[11] but both were back in place for the league game at San Mamés in late January, in which Maradona, still recovering from the injury, scored both his team's goals in their 2–1 victory.

[75][76] Upon the final whistle, Maradona (who had been insulted by Clemente in the build-up and was fouled repeatedly during the match, particularly by his marker Iñigo Liceranzu)[11][72][77] was goaded by Miguel Sola[9] and reacted by launching a flying kick.

[65] The 1986 Copa semi-finals again saw Athletic and Barcelona drawn together, and the second leg at San Mamés (won 2–1 by Barcelona)[79] saw incidents of objects being thrown by the crowd, a pitch invasion and insults made to the referee by Bilbao staff members, resulting in their stadium being closed for the next fixture, the first time this had occurred;[80] however, the hostile environment and disorder resulted largely from anger at the decisions and behaviour of the officials (who appeared to miss a foul during a Barça goal and failed to award a penalty to the home side)[79][80] rather than hostility towards the opposition.

[88] Following the 1996 Bosman ruling, the gap widened further; with no limitations on signing players from the European Union, clubs such as Barcelona could and did bring in the best talent from across the continent[89] – their squad in 1999, their centenary season included eight Dutch internationals as well as three Brazilians, an Argentine, a Portuguese and five who had represented Spain.

Entering the 21st century, Athletic endured some struggles with relegation while Barcelona also had a relatively disappointing period but then recovered and continued to grow increasingly more rich, popular and successful,[90] attracting several of the world's top players and drawing among the biggest crowds in Europe to their stadium.

[91][92] Under Frank Rijkaard, Barcelona won the Champions League in 2006 while also developing a highly talented group from their Cruyff-inspired youth system who would lead them to even greater success including three more continental titles, led by forward Lionel Messi, who broke virtually all of Zarra's longstanding records.

[100][8] During the post-match celebrations in both 2009 and 2012, Barça captains Carles Puyol and Xavi endeared themselves to the Athletic fans by parading the Ikurriña (Basque flag) alongside their Catalan Senyera[101][102] (in contrast to Andrés Iniesta who was unpopular due to accusations of exaggerating a foul leading to a red card in a league match),[33] while another connection between the squads in 2012 was the admiration openly expressed by Barcelona coach Pep Guardiola for the tactical methods of Marcelo Bielsa, his counterpart at Athletic Bilbao.

[8][16] It was their first trophy of any kind since 1984,[85] having endured a painful defeat to Atlético Madrid in the 2012 UEFA Europa League Final in addition to the losses to Barcelona, and so was celebrated with great enthusiasm by the supporters who crowded the streets of Bilbao city centre in their thousands for an open top bus parade normally reserved for much greater triumphs.

[107] The league results demonstrated a consistently wide gulf between the two clubs, with Athletic collecting no points at all from visits to the Camp Nou between 2004 and September 2018, when they finally secured a draw,[108] and only winning twice at San Mamés in the same period.

[113] As mentioned above, Athletic also eliminated Barcelona from that season's Copa del Rey by the same scoreline,[107] and the following year (by which time neither the winning coach in that tie Gaizka Garitano nor his defeated counterpart Quique Setién were in the dugout, replaced by Marcelino and Ronald Koeman respectively) they met in both the 2021 Supercopa de España Final, won by Athletic after extra time (at the end of which Messi was sent off for violent conduct for the first red card of his club career),[114][16] and then in the 2021 Copa del Rey Final, won 4–0 by Barcelona in a dominant performance from the outset, Messi scoring twice.

[118] In October 2017, a referendum was held in Catalonia on the subject of the region becoming independent from Spain, which the organisers claimed had resulted in the majority voting for independence; the process was declared illegal by the Spanish state both before and after it was held, with widespread disorder on polling day including violence by security forces against civilians in the city of Barcelona;[119][120] in the midst of this, FC Barcelona took the decision to close their stadium for a league fixture scheduled for the same day after a request to postpone it was rejected by the LFP – the match against Las Palmas was played behind closed doors.

In contrast, Athletic adhere to the club's Basque player policy and have experienced a similar decline in their stature to their male counterparts relative to the continued progression, professionalisation and globalisation of women's football, and the arrival of Real Madrid Femenino on the scene in 2020 ensured there would be another strong and well-financed opponent in contention for trophies[154] (although not ready to challenge Barcelona's dominance).