On July 27, 1919, white beachgoers in Chicago stoned a black teenager, Eugene Williams, whose raft had drifted past what was understood to be a racial dividing line in Lake Michigan near 29th Street. After Williams drowned, the cops decided to arrest an African-American for causing the trouble, and as word spread Chicago’s south side exploded into violence. For the next week, mobs of young men—often led from the white side by well-organized Irish gangs—crisscrossed the boundaries of the city’s “Black Belt,” committing beatings, shootings, stabbings, and firebombings that left 38 people dead and hundreds wounded, the majority of them black.

Most commentators, both at the time and in the decades since, have blamed the riot on racial tensions built up during World War I as hundreds of thousands of African-Americans moved to Chicago from the deep south. But that doesn’t explain why some of the same white gangs that stoked the 1919 violence had been terrorizing south-side streets—attacking Jews, Poles, even rivals from their own neighborhoods—for years.

Historian Andrew J. Diamond offers another theory: the incident, like so many others that shaped Chicago in the 20th century, was the product of a complex mix of racial and ethnic change, economic uncertainty, and young folks trying to figure it all out, often by beating each other bloody. In Mean Streets he draws on a deep well of research to tell the stories behind groups from Ragen’s Colts—a Bridgeport gang that specialized in sports, street thuggery, and vote production on election days during the 1910s and 1920s—to the Vice Lords, west-siders who branched out from gangbanging to black empowerment in the 60s, as well as the many nameless mobs and delinquents whose crude social protest over the years enforced the city’s color lines. It’s a fascinating and revealing narrative—though you’ve frequently got to parse heaps of academicspeak to find it. —Mick Dumke