The back story, for whatever narrative we can think of, establishes a context for the story – often a brief one – that is characterized as the main story.
With regard to gangs, the back story is rooted in history including military history.
Standard gangs rivalry story
A May 26, 2014 Toronto Star article is entitled: “Gangsters, goris and 10 cups of coffee: Life among the Dixon City Bloods: Project Traveller wiretaps offer rare glimpse into a world of alleged gun-running, drug dealing and bloody gang rivalries.”
The back stories
A May 6, 2014 blog post at this website provides a back story – a means of contextualizing the narrative – regarding the above-noted article. The post is entitled: “Alice Goffman’s ‘On the Run’ Studies Policing in a Poor Urban Neighborhood – New York Times, April 29, 2014.”
A related back story is highlighted in an August 6, 2013 blog post entitled: “The Drug Wars in America, 1940-1973 (Kathleen J. Frydl, 2013).”
The latter post has had some updates, as the Drug wars (2013) narrative is linked to many related topics:
A May 31, 2015 New York Times article is entitled: “Spike Lee Comes to Film ‘Chiraq,’ Unsettling Some Chicagoans.”
A June 19, 2017 London School of Economics and Political Science article is entitled: “Book Review: Sharing This Walk: An Ethnography of Prison Life and the PCC in Brazil by Karina Biondi.”