On a Sunday in early December 73 years ago the unthinkable, the totally unexpected happened--in a "day that will live in infamy" the Japanese bombed and destroyed the US Navy's battleship force in the Pacific.  But how could that have happened?

As the Naval History & Heritage Command states:

By late November 1941, with peace negotiations clearly approaching an end, informed U.S. officials (and they were well-informed, they believed, through an ability to read Japan's diplomatic codes) fully expected a Japanese attack into the Indies, Malaya and probably the Philippines. Completely unanticipated was the prospect that Japan would attack east, as well.

"U.S. officials (and they were well-informed)" and "completely unanticipated".  Hmmm, those phrasing combinations ring throughout history.

The U.S. Fleet's Pearl Harbor base was reachable by an aircraft carrier force, and the Japanese Navy secretly sent one across the Pacific with greater aerial striking power than had ever been seen on the World's oceans. Its planes hit just before 8AM on 7 December. Within a short time five of eight battleships at Pearl Harbor were sunk or sinking, with the rest damaged. Several other ships and most Hawaii-based combat planes were also knocked out and over 2400 Americans were dead. Soon after, Japanese planes eliminated much of the American air force in the Philippines, and a Japanese Army was ashore in Malaya.

These great Japanese successes, achieved without prior diplomatic formalities, shocked and enraged the previously divided American people into a level of purposeful unity hardly seen before or since.

As with September 11, as the years go by and those who lived through the shock of the Day of Infamy and its aftermath pass on the necessity for remembering remains. The Naval History site is a good place to start.