By far the most important event in the US that happened in the 1920s was the stock market crash of October 1929. Although the crash was not the true cause of the Great Depression, it is generally seen as the start of the Depression.
Some other things that happened in the 1920s and were very important were:
- The first radio broadcast. This was part of the change to a society that had more entertainments options and was centered around having fun. You could also put the first talking movie (1927) in with this.
- The Sacco and Vanzetti trial (1921) and execution (1927). These showed the fear of immigrants and of political radicalism that was part of this decade. You could also include the Immigration Act of 1924 here.
- The Washington Naval Treaties (1922). These were meant to bring about and maintain world peace. They were important parts of American isolation. You could put the Kellogg-Briand Pact (1928) here too.
- Scopes Trial. This showed the tensions between traditional ways and the new attitudes of the 1920s.