Why was the USA so unprepared for the attack on Pearl Harbor in 1941?

A statement by US President Truman on the outcome of the enquiry into Pearl Harbor, August 1945

Catalogue ref: ADM 199/1363

AUGUST 30, 1945

at his press and radio conference
August 30, 1945.

"I have read it (the Pearl Harbor reports) very carefully, and I came to the conclusion that the whole thing is the result of the policy which the country itself pursued. The country was not ready for preparedness. Every time the President made an effort to get a preparedness program through the Congress, it was stifled. Whenever the President made a statement about the necessity of preparedness, he was vilified for doing it. I think the country is as much to blame as any individual in this final situation that developed in Pearl Harbor."