Which event finally brought the U. S. into World War II?

The Japanese surprise attack on the naval base of Pearl Harbor in Hawaii, on December 7th 1941. Described by President Franklin Delano Roosevelt as "a date which will live in infamy" on his famous speech delivered the following day.

The attack happened in the absence of a declaration of war and without any kind of warning to the civilian population. Japanese attack planes sunk four battleships, severely damaged another four, sunk another two military boats, damaged three cruisers, three destroyers and three other vessels. It destroyed 188 planes and severely damaged 159 more. 2,335 servicemen were killed and 1,143 were wounded. 68 civilians were killed and 35 were wounded with 3 civilian planes shot down. The US declared war on Japan on December 8, one hour after Roosevelt's speech.