Answer:
When Germans using their U-boats on the British ships where lots of Americans were on board. Germany had promised they wouldn't use U-boats again but they did not keep their promise. Also, Germany sent a coded message to Mexico saying that they should work together and for Mexico to attack the US. When the US found out about this is when they could no longer keep neutral.
Explanation:
(By the way, I didn't know what you were talking about but I figured it was WWI)
