Why was Pearl Harbor attacked at the time it was?

Answer 1

Up until 1940 the US Pacific Fleet was based on the west coast of the continental US. President Roosevelt ordered the Fleet to remain in Hawaii, after the Fleet had maneuvers in Hawaiian waters in 1940. Roosevelt did this to send a signal to the Japanese of US concern over continued Japanese aggression, especially their invasion of China. The US had acquired Hawaii in the 1890s primarily as a "coaling station" for the coal fired, steam-powered ships of the navy. Admiral Kimmel commanded the US Fleet at the time of the Japanese attack on Pearl Harbor. Normally an admiral held this post for two years. The man who had the job before Kimmel, Admiral Richardson, probably lost his job (after only a year in command of the Fleet) for protesting too vigorously the moving of the Fleet's base to Pearl Harbor. Richardson (and Kimmel) saw all too clearly that it would be almost impossible to defend the Fleet while it was anchored in Pearl Harbor. Also, there was only one way in or out of Pearl Harbor, a single narrow channel, and if a ship was sunk in this channel it would prevent the ships of the Fleet from going in or out, trapping any ships in the Harbor until the sunken ship could be removed, which might take months.

Richardson began, and Kimmel continued, a pattern of regular Fleet operations. About half the Fleet would go out for a week, and return for the weekend, and the next week, the other half would go out. It was impossible to conceal these regular movements. The Japanese had a diplomatic Consulate in Honolulu. Every consulate of every nation, then and now, includes spies. Everybody knows this but as long as they stay out of restricted areas, they are free to go as they please and observe whatever they can see.

This is one of the true keys to the Pearl Harbor disaster. The US Fleet's regular schedule of operations allowed the Japanese to guess, correctly, as it turned out, that most of the US Fleet would be present in the Harbor on any weekend.

In one respect the US was very lucky - none of the US aircraft carriers were in the Harbor when the Japanese attackers appeared overhead.

Anti-Roosevelt politicians and historians have tried to make much of Roosevelt's decision to move the Fleet to this forward base in Hawaii, in support of their thesis that Roosevelt was trying to maneuver the Japanese into attacking the US, so the US could get into the war. This ignores the facts that the Japanese planned meticulously, innovated with genius, and trained very hard to make their attack a success. A task force which performed the stunning attack on Pearl Harbor could likely have done just as well if the Fleet was in Puget Sound or Long Beach, on the west coast.

Answer 2

Because the Japanese knew that if they did not give up their Empire the U.S. would have eventually declared war on them, and they knew that the U.S. had the industrial might to build its army and navy and air corps to one almost as big as theirs

Pearl Harbor was attacked on an early Sunday morning because the US Fleet had established a regular routine of operations, which saw most ships of the fleet returning to the Harbor for the weekend. The Japanese were aware of this regular routine and were able to guess correctly that a weekend attack would find most of the Fleet in the Harbor, and not much going on early on Sunday.

Pearl Harbor was attacked in early December because the Japanese could wait no longer. Their attack task force was to sail east from Japan across the northern Pacific until directly north of Hawaii, then turn south until within range to launch the attack planes. This course kept the task force away from regular shipping lanes, so they could hope to go undetected. The Japanese ships had short cruising ranges, and it was necessary to refuel all but a few of them at sea. Numerous tankers accompanied the Task Force for this purpose. The weather in the northern Pacific in winter is awful, frequently foggy, and the seas are tremendous, with huge waves. If they waited any later the weather would be too bad to allow at sea refueling, and they would have to wait for months. The original target date was for mid-November, but they were not yet ready.