With the Volstead Act taking effect on January 17th, 1920, the Prohibition Era in the United States began.
Bootlegging led to a rise in organized crime throughout the country, the Thompson Submachine gun and Browning Automatic Rifle were used regularly by criminal bands.
To combat the 24% increase in crime during the first 2 years of Prohibition, American police forces began to militarize, upgrading to automatic weapons and bigger calibers. When Prohibition ended, so did the militarization of the police, but this would only last for a few decades until race riots in the 1950s and 60s started and would continue to increase with the start of the War on Drugs in the 1980s.
Read the full history, and where we’re headed, on police militarization: Weapons of War On Our Streets: A Guide to the Militarization of America's Police at Ammo.com.