Prohibition in the United States, also known as "The Noble Experiment", was the period from 1919 to 1933, during which the sale, manufacture, and transportation of alcohol were banned nationally as mandated in the Eighteenth Amendment to the United States Constitution.
Under substantial pressure from the temperance movement, the United States Senate proposed the Eighteenth Amendment on December 18, 1917. Having been approved by 36 states, the 18th Amendment was ratified on January 16, 1919 and effected on January 16, 1920, which is when Season 1 for Boardwalk Empire begins.
The "Volstead Act", the popular name for the National Prohibition Act, passed through Congress over President Woodrow Wilson's veto on October 28, 1919, and established the legal definition of intoxicating liquor, as well as penalties for producing it. Agent Van Alden is trying to enforce the Volstead Act.
While Prohibition was successful in reducing the amount of liquor consumed, it tended to undermine society by other means, as it stimulated the proliferation of rampant underground, organized and widespread criminal activity, giving rise to such infamous historical figures as Enoch Lewis "Nucky " Johnson (the historical basis for main character Nucky Thompson), Al Capone, Lucky Luciano, and Arnold Rothstein.
Prohibition became increasingly unpopular during the Great Depression, especially in large cities, which included Atlantic City. The bulk of America became disenchanted after the St. Valentine's Day massacre in 1929. Until then, they felt that, even with setbacks, Prohibition was working.
On March 22, 1933, President Franklin Roosevelt signed into law an amendment to the Volstead Act known as the Cullen-Harrison Act, allowing the manufacture and sale of certain kinds of alcoholic beverages. On December 5, 1933, the ratification of the Twenty-first Amendment repealed the Eighteenth Amendment.