Start of Prohibition
Prohibition actually started much before the 18th Amendment, in 26 out of the 48 states at the time already had their own laws prohibition alcohol. One of the main reasons originally for the amendment was WWI and the rationing of products and the US need the materials used in the alcohol for the war efforts; thus the amendment banning alcohol in the United States was made and passed on January 29, 1919.
http://www.historicpatterson.org/Exhibits/ExhProhibition.php
http://www.historicpatterson.org/Exhibits/ExhProhibition.php