There were gangsters in every city and during the 1920s groups would fight each other for control of specific areas.
In Chicago, Dion O'Bannion controlled the bootleg business in the south of the city and John Torrio in the north. Al Capone was originally part of Torrio's gang before he took over. The Mayor of Chicago, 'Big' Bill Thompson, was under the influence of Torrio.
In New York, Dutch Schultz was in control.
Chester La Mare controlled Detroit.
The Saint Valentine's Day Massacre of 1929 was the climax of the gangster wars between Bugs Moran and Al Capone. The authorities could not find enough evidence to convict Al Capone. He was eventually found guilty of tax evasion in 1931 and sent to prison.