The Zimmerman telegram was a message from the German foreign minister asking Mexico for support if the U.S. entered the war, but, British intelligence discovered it. Since Great Britain and the U.S were allies, the British told the U.S.
I think that the number one reason for U.S. joining WW1 was because of the Zimmerman telegram.
When the president, Woodrow Wilson, found out, he released the captured telegram to the press.
I think the secondary reason the U.S. entered WW1 was because they were under attack.
The Americans were outraged, they wanted to join the fight.
The German government bought advertisement space in American newspapers warning that Americans who traveled on ships carrying war materials risked submarine attack, and felt that they had done their part to warn Americans about overseas travel.