The Mafia has a long history of corruption, murder, and selling illegal things. The Mafia biggest group was in New York and Chicago. The biggest time for the Mafia was during the 1920s.
The prohibition era made it illegal to have alcohol. The prohibition was put in place by women. The women did that because they did not want their husbands spending their free time at bars. Men would go to work and drink alcohol. Alcohol being illegal made the perfect opportunity for the Mafia to start and sell alcohol.