Final answer:
The 1927 U.S. Supreme Court decision confirmed that unlawful income, like the earnings of Al Capone from bootlegging and other crimes, was taxable, leading to his tax fraud conviction and imprisonment in 1931.
Step-by-step explanation:
The important event for Al Capone was the 1927 U.S. Supreme Court decision that upheld the Internal Revenue Service's contention that even unlawful income was subject to income taxes. This landmark ruling came at a time when Capone, a notorious gangster known for his bootlegging operations during Prohibition, was amassing a fortune through various illegal activities such as gambling, prostitution, and the sale of illicit alcohol. Despite his attempts to conceal the illegal origins of his wealth, the IRS pursued Capone for income tax evasion - a strategy that eventually led to his tax fraud conviction and imprisonment in 1931. The decision by the Supreme Court marked a significant moment in legal history, cementing the principle that all income, lawful or otherwise, is subject to federal taxation.