What pivotal event led the united states to enter world war ii

Japans attack on pearl harbor was the event that led to the U.S to enter World War 2.

the triple alliance. germany and austria-hungary were already allies by 1879 when, worried about a threat from russia, they formed the dual alliance. italy joined the group in 1882. there was a concern about a possible attack from france and the alliance needed powerful friends to defend it.