The significance of the 18th Amendment in the U.S. lies in its establishment of Prohibition, which prohibited the manufacture, sale, and transportation of alcoholic beverages. Ratified in 1919, it aimed to reduce crime and corruption associated with alcohol consumption. However, the amendment led to unintended consequences, including the rise of organized crime, bootlegging, and speakeasies, undermining enforcement efforts. The social unrest surrounding Prohibition eventually culminated in its repeal by the 21st Amendment in 1933, marking its failure as a social reform effort.