The attack on Pearl Harbor in 1941 was the determining factor that got the U.S. into WWII.
It was surprise attack by the Japanese on the major U.S. Naval base at Pearl Harbor. They attacked started at 7:48am, and continued for for two hours, until around 9:50am when the all the Japanese forces had returned to their carriers. The torpedo attack only lasted around 11 minutes, until bomber forces moved on the base. After the whole ordeal, 2,403 Americans were killed and 1,178 were injured. Out of he eight ships docked that day, four were sunk, the others severely damaged. All the ships were recovered however, except the USS Arizona. Six of them returned to active service.
The major part of why this attack was so deadly was because it was a surprise attack. U.S. forces were not ready for or expecting an attack, and by the time they had got their planes in the air, and anti-aircraft guns ready, the damage was already done.