Strategic Background: Why Pearl Harbor? 

By the beginning of the twentieth century, Japan's emergence as the leading power in the western Pacific made it a natural political and economic competitor of the United States of America. Rivalry between the two countries over commercial and territorial interests in the region grew from this time. As early as 1907, both nations could foresee the possibility of war with the other.

Any conflict, at least initially, would be a naval war. Japan realised that its navy was not, and never would be, the equal of the United States Navy. The Japanese expected the American fleet to move west and attack. To counter any such move, in the 1920s and 1930s, the Japanese planned a defensive strategy of attrition. Starting west of Hawaii, submarines, carrier-borne and land-based aircraft and light naval forces would attempt to destroy as many US ships as possible (up to 30% of the fleet it was hoped) as it sailed west before Japanese battleships moved in to win a decisive victory in home waters. When the Second World War broke out, the Japanese Navy enjoyed local superiority in the Pacific as America had not constructed the maximum number of ships allowed it by current international agreements. However, in the face of continuing Japanese aggression in Asia and crushing German victories in Europe, in July 1940 the USA decided upon a massive naval expansion. Within a few years, Japan's advantage would have disappeared. 

By 1940, the Japanese Army's campaign in China was making no progress. The navy offered an alternative strategy of a southward advance into Indo-China and the oil-rich Dutch East Indies. The execution of this policy from September 1940 onwards severely antagonised the USA and brought great risk of war. When, in July 1941, the US imposed a total oil embargo on Japan, the Japanese saw conflict as inevitable and began planning accordingly. 

It was in this context that Admiral Isoroku Yamamoto, Commander-in-Chief of the Combined Fleet, suggested an air attack on the US Pacific Fleet, which had moved from its usual base at San Diego on the American west coast to a mid-Pacific location at Pearl Harbor in May 1940. 

Yamamoto's plan was a development of the traditional Japanese defensive strategy. He gambled on a surprise attack to destroy the American naval capability in the Pacific, including its all-important aircraft carriers, and create enough time, perhaps six months, to enable Japan to complete its territorial conquests. Simultaneous attacks by the army on Hong Kong, Malaya, the Philippines, Guam and the Dutch East Indies would capture the strategically important bases and areas rich in raw materials Japan felt was vital for its national survival and would also now be needed to sustain its war with America. A long struggle was expected, but it was hoped that the inevitable American onslaught would founder on the fortified defensive ring the Japanese would create around their empire.

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