The main purpose of the anti-saloon league in the early 1900s was

to pass a constitutional amendment outlawing alcohol.

Explanation:

The Anti-Saloon League was a nationwide organization that had as its main purpose to pass a constitutional amendment outlawing alcohol. Deeply influenced by religious ideas, it was first founded in 1893, and by the early 1900s was the leading organization lobbying for prohibition of alcohol in the United States. Its main purpose was achieved when the Eighteenth Amendment was ratified on early 1919. However, this amendment was repealed with the passage of the Twenty-First Amendment in late 1933, ending Prohibition in the United States.