That's exactly the ages old problem. We might aspire to never letting our guard down BUT it's always the new unforeseen threat that overcomes that overcomes that guard. In the Pearl attack, it's not exactly unforeseen. It's been foreshadowed by the British attack on Taranto, which the Japanese did study. The problem was that some US officers knew about this too but their views were dismissed.
The Pearl Harbor attack, I think, is a case study of an intelligence problem. Intelligence had indicated that Japan will attack and senior US officials knew this. But if the intelligence gained is fully relayed to the full organization, it would reveal to Japan that their codes were compromised. A balance needed to be gained in this situation but unfortunately for the US, circumstance caused it to slip and lose balance.
Personally, I still don't believe that there was a conspiracy at all about the attack on Pearl Harbor.