The St. Valentine’s Day Massacre stunned the world on February 14, 1929, when Chicago’s North Side erupted in gang violence. Gang warfare ruled the city during the late 1920s, as chief gangster Al Capone sought to gain control over the illegal sales of alcohol and other vices by eliminating his rivals.

SOURCES

The St. Valentine's Day Massacre, William J. Helmer Jr. and Arthur J. Bilek
https://vault.fbi.gov/St.%20Valentines%20Day%20Massacre
https://webharvest.gov/peth04/20041019072541/http://foia.fbi.gov/stvalen/stvalen2.pdf