The Mod-Rocker Wars were a curious cultural phenomenon in Britain in the 1960s, where two slightly different youth subcultures with marginally divergent preferences in such things as art and music ended up, somehow, as bitter enemies who got into physical clashes. This appears to be an instance of the human tendency to make enemies of people who differ in an exceedingly minor way, such as the troubles between Catholics and Protestants, who, as slight variations on Christianity, are more positioned to strongly oppose one another than either is for a totally different religion such as Zoroastrianism.
In the so-called "real world", this conflict (Mods/Rockers, not Protestants/Catholics) was over and done with by the end of the '60s, but in the Tackleford universe it had an even more inexplicable resurgence in the 2010s, as recounted in The Case of the Modern Men.
In Tackleford, a "mod" subculture apparently managed to continue up to the present day, with a whole rulebook full of traditional practices and methods of winning leadership of the group through challenges. This group of mods most likely stayed beneath the radar of popular culture by being extremely small and unpopular for most of the intervening decades, until there was a sudden resurgence of popularity. Jack Finch claimed credit for this, but even if this is true, he didn't end up in the forefront of the movement he spearheaded, as it was rapidly taken over by more popular kids. A sizeable group gathered regularly beneath a bridge to show off their motor scooters.
Meanwhile, a "rocker" subculture also had an upsurge of popularity in Tackleford, with various "garage bands" showing off new musical genres such as "Power Violence". Shauna Wickle joined one such band, Brutalism UK, in the mistaken impression that it was a group celebrating mid-20th-century architecture.
With the two cultures so established, they soon began clashing, with violent battles doing damage to downtown Tackleford and leading the local police to impose a curfew. This didn't stop the battles from continuing, until a masked leader Yumi (who later revealed herself to be Shauna) brought about a truce ending the war.
You can read more about the real-world mods and rockers here.