Prohibition summary The noble experiment of Prohibition was introduced by the 18th Amendment, which became effective in January 1920. Here are four reasons why Prohibition was introduced: 1  National mood - when America entered the war in 1917 the national mood also turned against drinking alcohol. 2  Practical - a ban on alcohol would boost supplies of important grains such as barley. 3  Religious - the consumption of alcohol went against God's will.