The attack on Pearl Harbor was an important lesson for the United States.
Among so many other things learned, probably the most important was to not discount any gathered intelligence until it was investigated and found to be of no value. Sadly, we had intel of the impending attack but it was deemed unreliable before it was actually vetted.
Additionally, while I commend our leaders for not wanting to enter into another war unless it was absolutely necessary, there was enough evidence that we would eventually have to fight. By delaying our involvement we allowed the Axis powers to grow in strength and secure positions that eventually cost many American lives to reclaim.
While the attack on Pearl Harbor set us back in the Pacific, militarily speaking, it served as a motivating catalyst to enter the war with determination. This would prove to be essential, particularly against the Japanese, as they were ruthless and determined to win at any cost.