Why were the Japanese so successful in the attack on Pearl Harbor?

I'm not
sure that the U.S. had underestimated the Japanese; in fact American intelligence had broken the
Japanese code and were constantly intercepting messages sent from Tokyo to the Japanese
ambassador in Washington. There would have been no reason to keep such a close eye on Japanese
activity had the Japanese been underestimated. The code breaking program, known as
"Operation Magic" had indicated that something dire was afoot. The last decoded
message, indicating that the ambassadors were to deliver a final message at a precise time
(actually thirty minutes before the planned attack) also indicated that they were to destroy all
decoding equipment...