Chicago
The ratification of the 18th amendment to the Constitution in 1919 made the production and sale (including exportation) of alcoholic beverages illegal in the United States. This ushered in the beginning of what is known as the Gangster Era, a time that roughly spans from 1919 until 1933 when Prohibition was repealed. The 1920s saw gangsters, including Al Capone, Dion O'Banion, Bugs Moran and Tony Accardo battle law enforcement and each other on the streets of Chicago during the Prohibition era. Chicago was the location of the infamous St. Valentine's Day Massacre in 1929, when Al Capone sent men to gun down members of a rival gang, North Side, led by Bugs Moran.
American Mafia
On January 17, 1920, Prohibition began in the United States with the 18th Amendment to the United States Constitution making it illegal to manufacture, transport, or sell alcohol. Despite these bans, there was still a very high demand for it from the public. This created an atmosphere that tolerated crime as a means to provide liquor to the public, even among the police and city politicians. Not explicitly related to Mafia involvement the murder rate during the Prohibition Era rose from 6.8 per 100,000 individuals to 9.7 and within the first three months proceeding the Eighteenth Amendment, a half million dollars in bonded whiskey was stolen from government warehouses. The profits that could be made from selling and distributing alcohol were worth the risk of punishment from the government, which had a difficult time enforcing prohibition. There were over 900,000 cases of liquor shipped to the borders of U.S. cities. Criminal gangs and politicians saw the opportunity to make fortunes and began shipping larger quantities of alcohol to U.S. cities. The majority of the alcohol was imported from Canada, the Caribbean, and the American Midwest where stills manufactured illegal alcohol.