With control of the Philippines the prize, the vastness of the Pacific gave the Japanese Imperial Navy a significant advantage over the United States Navy. The main base of the United States Pacific Fleet between 1898 and 1940 was San Diego on the American West Coast. The distance from San Diego to the Philippine capital Manila is 7,254 miles (11,700km). The distance between San Diego and Pearl Harbor is 2,455 miles (3,960 km).By comparison, the distance from the port of Yokohama on Tokyo Bay to Manila is only 1,953 miles (3,150km). These distances, and the related problem of logistics, dominated American and Japanese naval planning from 1898 to 1945.

Aircraft carriers like the fast and powerful Akagi enabled Japan to project its military power across the vastness of the Pacific Ocean. Akagi was the flagship of Vice Admiral Chuichi Nagumo for the treacherous surprise attack on the United States Pacific Fleet at Pearl Harbor. Originally laid down as a battle cruiser, Akagi was converted to an aircraft carrier as a result of the Washington Naval Treaty 1921-22.

The Japanese were very conscious of the disparity in industrial strength between Japan and the United States. In the 1930s, Japan's industrial output could be equated with that of Italy. In a drawn out war with the United States, Japan must eventually lose. If war with the United States could not be avoided, Japanese military planners banked on a bloody war of attrition wearing down American will to a stage where the American government would welcome peace talks with Japan.

Japan acquires a chain of islands between Hawaii and the Philippines during WW I

During World War I, at little risk to itself, Japan aligned itself with the Allied cause, took control of German commercial holdings in China, and occupied the German-owned Marianas, Caroline and Marshall island groups which lie in the Pacific Ocean between the Philippines and Hawaii. At the end of World War I, despite strong opposition from the United States and Australia, the newly formed League of Nations granted Japan trusteeship or Mandates over these islands. To ease American concerns, the Japanese undertook not to fortify these League of Nations island Mandates in return for an undertaking from the United States that it would not modernise its defences in the Philippines.

Japan now had an empire stretching from Korea on the Asian mainland to the central Pacific, and had replaced Russia as the dominant foreign influence in China's northern Manchurian region. To back its expansionist foreign policy, and with British help, Japan had created the third largest navy in the world after those of Britain and the United States. However, the Japanese Navy now had to grapple with a problem that had dominated US Navy planning for the defence of the Philippines. That problem was the distance between Japan and its Pacific island Mandates. The distance from Yokohama to the Japanese Navy anchorage at Wotje in Japan's Marshall Islands Mandate is 2,566 miles (4,140km). The distance between Pearl Harbor and Wotje is 2,198 miles (3,546km). Small contingents of the Japanese fleet located at Truk in the Carolines and Wotje in the Marshalls not only had to be kept supplied across vast distances, but they were at risk of being cut off and overwhelmed by a larger American naval task force from Hawaii before reinforcements could arrive from Japan.

While the American naval war planners worried that fortification of the Japanese Mandate island groups could block their lines of communication with the Philippines in the event of war with Japan, the Japanese naval war planners worried that the United States Navy might seize each of their far distant island Mandates in turn and use them as stepping stones to reach the Philippines and Japan's home islands. The stepping stone advance through the Japanese Mandates was in fact one of two Pacific war scenarios being seriously considered by the American naval war planners. The other scenario was a bold and swift fleet to fleet confrontation with the Japanese Navy that could provide fast relief to the beleaguered American garrison on the Philippines.

As it turned out, the disaster at Pearl Harbor on 7 December 1941, and the adoption by the United States and Britain of a "Germany First" war strategy at the Arcadia Conference in late December 1941, left the US Navy in 1942 with only the first option.

Naval Limitation in the Pacific

The British were concerned about Japan's intentions towards their colonies of Hong Kong, Malaya and British Borneo. The Americans were deeply concerned that Japan's newly acquired central Pacific island possessions lay across their lines of communication between the Philippines and Hawaii. To meet this potential threat, the Americans reorganised their navy into Atlantic, Pacific and Asian fleets, and stationed their most powerful battleships in the Pacific.

With the intention of avoiding a naval arms race with the Japanese, and reducing the potential for naval conflict in the Pacific, the Americans convened the Washington Naval Conference in 1921-22. The participating naval powers, including Japan, agreed to a tonnage ratio of 5:5:3 for large warships of the United States, Great Britain and Japan respectively. The effect of the Washington Treaty was that the United States and Britain would each have fifteen battleships to nine of Japan. For aircraft carriers, the ratio was the same, with a cap of 135,000 tons for the United States and Britain, and 81,000 tons for Japan. The Japanese were not permitted by the treaty to offset the numerical limitation imposed on them by increasing the calibre of naval guns.

The apportionment of ship tonnage produced by the Washington Treaty was intended to prevent any one nation becoming a dominant naval power in the western Pacific, and recognised the fact that Britain had to protect an empire that spanned the globe, and that the United States had to protect lengthy coastlines on two oceans and the Philippines. Despite these considerations, the lower naval tonnage allocated to Japan by the Washington Treaty was deeply resented by militarists in Japan who viewed it as a humiliation imposed on Japan by the United States and Britain. The terms of the Washington Treaty were extended to 1936 by the London Naval Treaty of 1930.

In response to the treaty limitations, the Japanese Navy plans for a defensive war of attrition against the United States

The warship limitations imposed by these naval treaties meant that Japan could not readily undertake an aggressive naval war against the United States. As a response to the limitations, and the greater battleship strength of the United States Navy, Japanese naval planners turned to developing defensive strategies that were deemed likely to produce a Japanese victory in the event of war with the United States. It was recognised that the likely trigger for such a war would be a Japanese attempt to seize the Philippines. Japanese naval strategy envisaged that the Imperial Navy would initially fight a defensive war against the United States Navy. The purpose of the initial defensive phase was to draw an American fleet close to Japan or the Philippines where attrition by submarine and land-based bomber attacks and concentration of the strength of the Japanese Navy would enable the American fleet to be destroyed. The Japanese believed that destruction of the American fleet could be a decisive factor in persuading the United States to accept a compromise peace.

Tactics were also developed to counter the greater battleship strength of the United States Navy. The Japanese Navy planned to fight at night and avoid daytime naval actions. A great deal of effort was put into developing night warfare skills and building fast, lightly armoured cruisers and destroyers equipped with highly accurate, long-range and very deadly Type-93 torpedoes. As aircraft technology advanced, and the long-range bomber was developed, this reinforced the thinking of Japanese naval planners that the decisive stage of a conflict with the US Pacific Fleet should be fought close to Japan.

American naval planners recognise the vulnerability of their Philippine garrison to capture by the Japanese

The American navy planners were acutely aware of the problems that an American fleet would experience in fighting its way to the Philippines against determined Japanese naval opposition. Throughout the 1930s, the main base of the US Pacific Fleet was at San Diego on the American West Coast, and the distance from San Diego to the Philippine capital Manila is 7,254 miles (11,700km). Pearl Harbor was then little more than a way station to the Philippines, and it lacked the facilities to sustain a fighting advance to the Philippines by the US Navy. Fleet exercises in the 1920s and 1930s demonstrated repeatedly that the US Pacific Fleet did not have enough ships to overcome the Japanese Navy in its home waters, and this problem was compounded by the failure of successive administrations and Congress to modernize the American battleship fleet or even keep it at a strength allowed by the naval treaties. Although never mentioned to the American public, US Navy planners realised that an American garrison in the Philippines would be likely to overwhelmed by the Japanese before an American fleet could come to its rescue.

Recognition of the decisive power of the aircraft carrier

During the 1930s, both the American and Japanese navies began to appreciate that the striking power of aircraft carriers could be greater than that of battleships. Admiral Isoroku Yamamoto witnessed demonstrations of carrier power when attending American fleet exercises and became a strong supporter of naval aviation. He helped mould the powerful carrier striking forces that enabled Japan to dominate the seas during the first six months of the Pacific War. The Japanese carriers and their air groups were able to hone their combat skills during Japan's brutal war against China that began in 1937, and this gave them a significant advantage over their American counterparts during the first twelve months of the Pacific War.