By the early 20th century, Italian criminal gangs had begun forming in the northeast, most notably in New York City, as the US became increasingly attractive to Italian immigrants, competing with South America as a destination of choice. They were small-scale operations of no particular distinction to set them apart from other NYC gangs, generally operating in Italian neighborhoods and to preying upon Italian immigrants.
That all changed in 1920, after the passage of the 18th Amendment and the introduction of Prohibition, banning the manufacture, transport, or sale of alcohol. Making alcohol illegal did not reduce the high demand for alcohol, however, creating an environment of widespread tolerance of crime in order to provide a thirsty public with the alcohol it craved. Essentially, Prohibition took a major American industry, hitherto operated legally, and gifted it to organized crime.
The profits that could be made from the sale and distribution of alcohol were astronomical, and that, coupled with widespread tolerance by much of the public, as well as many cops and politicians who did not see the sale or consumption of alcohol as particularly venal or morally blameworthy, made bootlegging irresistible to criminal entrepreneurs across the US.
Prohibition thus gave a boost to organized crime in general, and to the Italian mafia in particular, as Italian gangsters were particularly well-positioned to take advantage of this new environment. Unlike other ethnic criminal entities, Italian criminals, thanks to the mafia in the old country, could readily draw upon a tradition of sophisticated, organized, hierarchical, and disciplined criminal organizations. They not only had the model but also experienced personnel who could readily duplicate the system in the US.
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