During the period between the passage of the 18th amendment and the ratification of the 21st (repealing the 18th), the buying and selling of liquor was illegal in the United States. (The Volstead Act in 1919 provided for the enforcement of the 18th amendment, and went into effect in 1920.)
Prohibition arose out of a long tradition of temperance movements in American history, and its causes are manifold. Still, strong anti-Catholic sentiment in the post-World War I era, which identified Catholics with the cause of the "wets," played an important role. Besides the fact that wine and beer were integral parts of the cultural traditions of Irish, Italian, German, and other Catholics, many Catholics also perceived the anti-Catholic tenor of the "dry" cause and it was resisted strenuously in many Catholic communities.
Identified as it was with the Republican Party, Prohibition had important political ramifications, helping to cement the ties between the majority of Catholics and the Democratic Party. When Catholic Al Smith ran for president in 1928, he was attacked, among other reasons, for his lack of support for Prohibition.