The Saint Valentine's Day Massacre was a notorious gangland killing which occurred on Valentine's Day in 1929. It was meant to be the culmination of a bitter struggle for the control of Chicago between two rival gangs, but turned out to be unsuccessful. Not only did the target of the Saint Valentine's Day Massacre fail to show up, but the massacre attracted so much public attention that it led to the ultimate downfall of infamous Prohibition-era mobster Al Capone.
Prohibition-era Chicago was certainly familiar with mob violence, thanks to the fact that it was conveniently located for the purpose of smuggling alcohol and other black market goods, but the Saint Valentine's Day Massacre shocked even Chicago's jaded citizens. Although a number of gangs and organizations operated in Chicago in the 1920s, two struggled for supremacy: Al Capone's South Side gang, and Bugs Moran's North Side gang. The battles between these rival gangs also illustrated the racial tensions of the 1920s, as Al Capone supervised an Italian gang, while Bugs Moran headed a mixed Irish and German gang.
At some point in 1929, Al Capone decided that it was time to eliminate Bugs Moran, and he delegated the job to one of his lieutenants while he vacationed in Florida to establish an alibi. A four-man team engineered the Saint Valentine's Day Massacre, luring representatives of Bugs Moran's gang to a warehouse in the hopes of ensnaring Moran as well. The nature of the lure used is not known; the massacred men were very well dressed, suggesting that they didn't arrive at the warehouse to pick up a shipment of liquor, but perhaps to do business.
Once the men arrived in the warehouse, two assassins dressed as policemen entered the building, and pretended to be on a routine police sting. After disarming the seven victims and forcing them to stand with their backs to the room, the “policemen” stepped back and allowed two men in plain clothes to mow the victims down. To allay suspicions about the gunfire, the faux-policemen led the men in plain clothes out at gunpoint and drove them away, staging a neat getaway and delaying the eventual phone call to the actual police.
Five of the men killed in the Saint Valentine's Day Massacre were definitely in Moran's gang, and another was a follower. The seventh appears to have been an innocent mechanic who was simply in the wrong place at the wrong time. Moran himself was not at the site; witnesses suggested that he had seen the men dressed as police and fled.
It took almost the entirety of 1929 to break the case, which attracted a great deal of attention. Thanks to the events of the Saint Valentine's Day Massacre, the federal government felt like it needed to make a move on Al Capone, ultimately bringing him down through the Internal Revenue Service, which prosecuted him for failure to pay taxes.