From Dave McGowan's Understanding the F-Word
The ostensible reason for our entry into the war, it will be remembered, was the atack on Pearl Harbor on December 7, 1941. Strangely enough, this attack came the morning after a massive counterattack was launched by the Red Army to repel the hordes of Nazi troops running amok through much of the western half of the Soviet Union.
Prior to this counteroffensive, it was widely assumed that the Germans would soundly defeat the Soviet forces. And the Americans didn't much care. They didn't really want to get involved, just as they didn't want to get involved as the German troops had plowed through Czechoslovakia, Poland, Hungary, Romania, Bulgaria, Yugoslavia, Belgium, France, Holland, Greece and Norway, as they set about creating a continuous 1800 mile front on which to attack the Soviet Union.
With the launching of the Soviet counteroffensive, however, America suddenly took a keen interest in the affairs of its European neighbors. It suddenly became apparent that the Red Army, unlike most of those previously encountered by the Axis forces, was actually going to fight back and, most likely, ultimately defeat the German Army.
And if that were to happen, our Russian 'allies' might even decide to roll on through Western Europe, doing a more thorough job of denazifying the region than some Westerners had in mind. So it was a good thing that the Japanese launched such a timely attack on Pearl Harbor, giving the US the necessary pretext for jumping into the fray.
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Prior to the unexpected Soviet counteroffensive, a fierce battle had been waged to steer public opinion away from supporting US entry into the war. This effort was led by such notables as Charles Lindbergh, Father Caughlin and Henry Ford, and organizations such as America First. The net result was that a major event was required to swing the pendulum of public opinion in the other direction--and quickly.
The timing of the attack, then, is suggestive not just of US awareness, but quite likely of US complicity. In any event, the sequence of events leading up to, and on the day of the attack certainly doesn't suggest that it came as much of a surprise. From a mainstream account written at the time, here is what occurred:
Late August 1941--Kichisaburo Nomura was dispatched to Washington to begin US/Japanese talks.
September 21--The talks reportedly reached a deadlock.
November 15--Talks resumed with negotiator Saburo Kurusu having been added to the Japanese team.
December 3--Negotiations again reportedly reached a deadlock.
December 7--Kurusu and Nomura resumed talks with US Secretary of State Cordell Hull in Washington. Meanwhile, at Pearl Harbor, Hawaii, a US Navy supply ship spotted a Japanese sub at 6:30 AM and ordered an air-strike on the location, but the crew did not sound an alarm. At 7:02 AM, Corporal Joseph Lockard--manning Pearl Harbor's aircraft detection system--spotted a large fleet of Japanese aircraft just 130 miles northeast of Oahu. His superiors inexplicably dismissed the fleet as US planes.
At 7:45 AM, another Japanese sub was spotted, this one having penetrated the submarine nets protecting the base. There was no US response. Ten minutes later, even as meetings were being held in Washington, the first bombs struck the naval base in a sustained attack allegedly intended to disable the Pacific Fleet. Strangely though, shore installations and oil storage facilities were not targeted. And conveniently, all three US aircraft carriers--far and away the most valuable assets in the fleet--were out to sea.
Three-and-a-half hours after the initial attack, a second 'surprise' attack occurred at 11.29 AM, followed by another at 11:59 AM. These were followed by further 'surprise' attacks at 12.22 PM, 7:15 PM and 9:10 PM, although--after awhile--the surprise factor must surely have beeen wearing a bit thin. Most of these later attacks seemed more concerned with bombing civilian centers and strafing the streets with macine-gun fire than with inflicting further damage on the naval base.
This obviously served no military purpose, but did serve to enrage the American people. In the end, the damage to the Pacific Fleet was relatively minimal. Six of eight battleships survived, along with--as previously noted--all three aircraft carriers and virtually all the shore installations vital to keeping the ships operational, leaving America very well-equipped to enter the war.