Final answer:
The widespread use of morphine among soldiers during the Civil War began a trend of addiction, but it was Prohibition that really catalyzed drug sales and organized crime, leading to the rise of street gangs involved in the drug trade.
Step-by-step explanation:
The invention of morphine and its widespread use during the Civil War is often cited as a turning point in creating widespread addiction among soldiers. However, it was during the Prohibition Era and the illegal trade of alcohol that gave rise to significant interest in drugs sales and the underground market. This period saw an increase in organized crime and the emergence of street gangs involved in the distribution of illicit substances. In the post-war period, especially after the First and Second World Wars, many cities experienced a surge in vice due to the influx of soldiers from rural areas who were exposed to urban life and, potentially, substance abuse. The situation was exacerbated by prohibition policies that created a lucrative black market for alcohol, which was soon controlled by crime syndicates. This later transitioned into the illegal narcotics trade and grew through the 20th century with the introduction of new drugs such as crack cocaine in the 1980s, further complicating law enforcement efforts and fostering urban gang violence.