From Wikipedia

Crime Inc.
Episode Three

Racket Busters

Mafia activities were restricted until 1920, when they exploded because of the introduction of Prohibition. An example of the spectacular rise of the Mafia due to Prohibition is Al Capone's syndicate that "ruled" Chicago in the 1920s.

In New York City, by the end of the 1920s, two factions of organized crime had emerged, causing the Castellammarese War for control of organized crime in the city. With the murder of Joseph Masseria, the leader of one of the factions, the war ended uniting the two sides back into one organization now dubbed Cosa Nostra. Salvatore Maranzano, the first leader of American Mafia, was himself murdered within six months, and Charles "Lucky" Luciano became the new leader. Maranzano had established the code of conduct for the organization, set up the "family" divisions and structure, and established procedures for resolving disputes. Luciano set up the "Commission" to rule their activities. The Commission included bosses from six or seven families.