There was only one event that led the US to enter WWII. That was the Japanese attack on Pearl Harbor in what was then the Territory of Hawaii. That happened on December 7, 1941. No other event actually caused the US to participate in the war.
the answer is 4: The attack on pearl Harbor by Japanese bombers on December 7, 1941 which sunk 18 U.S. Ships and destroyed 188 U.S. Aircraft and killed 2,403 men.
