The attack on Pearl Harbor was anything but a last-minute decision by the Japanese military. In truth, the assault was more than a year in the planning, and was under way even as the Japanese government went through the motions of negotiating with the United States regarding Japan's ongoing aggression in the Pacific.

The mastermind behind the attack on Pearl Harbor was Admiral Isoroku Yamamoto, commander in chief of the Japanese Combined Fleet. Yamamoto began planning the attack in November 1940, just two months after signing the Tripartite Pact, which brought Japan into the Axis with Germany and Italy.

It is commonly believed that Yamamoto had two sources of inspiration for the attack on Pearl Harbor. The first was a 1925 book titled The Great Pacific War by Hector Bywater, a British naval expert. Bywater's book realistically describes a war between the United States and Japan that begins with the destruction of the Pacific Fleet and a Japanese invasion of the Philippines and Guam. The second source was the November 1940 RAF strike at Taranto, Italy, in which torpedoes heavily damaged two Italian battleships. The event impressed Yamamoto because it was the first practical attack by aircraft on battleships.

In January 1941, Yamamoto instructed Rear Admiral Takijiro Onishi, a renowned naval aviator, to prepare a preliminary study on the feasibility of an attack on the U.S. naval base at Pearl Harbor. The plan was completed in April and called for Japan to make a surprise air and submarine attack on Pearl Harbor. The site was selected because it was both a major U.S. military installation and close enough for Japanese forces to approach with little risk of detection.

Commander Mitsuo Fuchida, director of the air attack, spent the next several months developing the tactics and equipment necessary to fly to Hawaii and take out the ships along Battleship Row. In November, after the Japanese government approved the attack, it became known as the “Hawaiian Operation.”

On November 30, the Japanese cabinet set December 7 as the date Japan would declare war on the United States. On December 1, Yamamoto's flagship in Japan's Inland Sea sent out a prearranged coded message — “Climb Mount Niitaka” — which was the signal to attack Pearl Harbor on Sunday, December 7.

Victory Breeds Confidence

Pearl Harbor was not the only target of the Japanese on that fateful day. Simultaneously, Japanese planes attacked the British in Malaya and Hong Kong and American military bases in the Philippines, Guam, Midway, and Wake Island. The barrage caught everyone by surprise, and there was little time to mount a defense. Within twenty-four hours, a huge expanse of the Pacific belonged to Japan.

Yamamoto and Tojo both believed the war was over at that point, confident that the United States and Great Britain had been weakened to the point where they could not and would not fight back. Hitler, too, believed that the United States would not pursue war, telling his aides, “Now it is impossible for us to lose the war. We now have an ally who has not been vanquished in 3,000 years.”

Figure 3-2 Roosevelt signing the Declaration of War against Japan the day after the bombing of Pearl Harbor.

Photo courtesy of the National Archives (79-AR82)

Lulled by America's earlier neutral stance and now believing the nation to be weak and essentially defenseless, Hitler declared war on the United States on December 11, 1941. Italy followed suit shortly after. But rather than rolling over as anticipated, the United States reacted almost immediately with its own declarations of war against Germany and Italy, while Great Britain declared war on Japan.