Today, Dec. 7, 2008, is the 67th anniversary of the attack on Pearl Harbor. Dubbed "a day that will live in infamy" by then-President F. D. Roosevelt, it confirmed that the US would join in the action in World War II:
The strike was intended to neutralize the U.S. Pacific Fleet, and hence protect Japan's advance into Malaya and the Dutch East Indies, where Japan sought access to natural resources such as oil and rubber. Both the U.S. and Japan held long-standing contingency plans for war in the Pacific which were continuously updated as tensions between the two countries steadily increased during the 1930s, with the Japanese expansion into Manchuria and French Indochina greeted by steadily increased levels of embargoes and sanctions from the United States and other nations.
In 1940, under the authority granted by the Export Control Act, the U.S. halted shipments of airplanes, parts, machine tools, and aviation gasoline, which was perceived by Japan as an unfriendly act. The U.S. did not stop oil exports to Japan at that time in part because prevailing sentiment in Washington was that such an action would be an extreme step, given Japanese dependence on U.S. oil, and likely to be considered a provocation by Japan.
Following Japanese expansion into French Indochina after the fall of France, the U.S. ceased oil exports to Japan in the Summer of 1941, in part because of new American restrictions on domestic oil consumption. President Franklin D. Roosevelt had earlier moved the Pacific Fleet to Hawaii and ordered a military buildup in the Philippines in the hope of discouraging Japanese aggression in the Far East. As the Japanese high command was (mistakenly) certain any attack on the United Kingdom's Southeast Asian colonies would bring the U.S. into the war, a preventive strike appeared to be the only way for Japan to avoid U.S. naval interference. An invasion of the Philippines was also considered to be necessary by Japanese war plans, while for the U.S., reconquest of the islands had been a given of War Plan Orange in the interwar years.
While the attack accomplished its intended objective, it was completely unnecessary. Unbeknownst to Isoroku Yamamoto, who conceived the original plan, the U.S. Navy had decided as far back as 1935 to abandon 'charging' across the Pacific towards the Philippines in response to an outbreak of war (in keeping with the evolution of Plan Orange). The U.S. instead adopted "Plan Dog" in 1940, which emphasized keeping the Imperial Japanese Navy (IJN) out of the eastern Pacific and away from the shipping lanes to Australia while the U.S. concentrated on defeating Nazi Germany.
The attack was an important engagement of World War II. Unintentionally occurring before a formal declaration of war (which had been scheduled to be delivered shortly prior to the attack beginning), it pushed U.S. public opinion from isolationism to the acceptance of participation in the war being unavoidable. The lack of warning led Roosevelt to call it "a date which will live in infamy."
Much has been written about Pearl Harbor, including that FDR, etc., may have been forewarned. Of course, the same allegations were made regarding 9/11, especially after it was shown that GWBush received a PDB titled "Bin Ladin Determined To Strike in US" which was largely ignored.
So 9/11 wasn't the first attack on the US, and likely won't be the last. Let's hope that the next won't lead to global conflict like WWII, and won't lead to ideological missteps like attacking Iraq.