In 1917 World War I began. America tried to stay neutral but couldn't due to these two main reasons:
Germans continuing to practice unrestricted submarine warfare and sinking frieghters and tankers without warning- breaching the “rules” of war.
Germany getting caught sending a telegram trying to become allied with Mexico to gain access to United States borders and to provide Mexico with funds to gain back land lost to the United States (NM, TX, AZ).