Remember, World War 1 was the first of two wars, which started on July 28th, 1914 and ended on November 11th, 1918. A major factor that led the United States to enter World War 1 would be because of Germans attacking American ships that had Americans on it, using submarines. Another reason would be because of a British telegram, called the Zimmerman Telegram/Note, that offered an alliance between Germany and Mexico.
Two major factors that caused the U.S to join WWI, was the Zimmerman Telegram, and Germany's announced intention to resume unrestricted submarine warfare. I hope I could help :)!
