Japan declares for itself a "Greater East Asia Co-Prosperity Sphere"

As Japan's war in China continued through 1938, the United States hoped that the mounting costs of the seemingly unending war would cause Japan to review its policy towards China and withdraw its troops. When this did not occur, the Americans began to take economic measures against Japan. Those measures included denial of certain war-related raw materials, termination of trading privileges, and assistance to China's Nationalist government at Chungking.

These economic measures only succeeded in hardening the attitudes of Japan's militarists, and increasing their hostility towards the United States, which they felt was meddling in Japan's natural sphere of interest in East Asia. The Japanese imperial government responded to the American economic measures by declaring its intention to found a "New Order" in Asia by establishing a Greater East Asia Co-Prosperity Sphere.

The New Order would involve a Japanese-controlled political and economic grouping of countries for the purpose of supplying Japan with the raw materials needed by Japan's industries, and in particular, its war-related industries, and accepting Japanese exports in return. The list of countries included in Japan's New Order included Australia, Burma, the Netherland's East Indies (now Indonesia), Malaya, New Guinea, New Zealand, the Philippines, and Thailand. The Japanese made it quite clear that any country resisting inclusion in their Greater East Asia Co-prosperity Sphere would be treated as an enemy of Japan.

By signing the Tripartite Pact on 27 September 1940, Japan allied itself with the Axis Powers, Nazi Germany and Fascist Italy, in a partnership for world domination by military aggression.

As tensions increased between Japan and the United States, the Japanese began looking for allies who would support aggression by Japan.

Hitler and Mussolini bring Japan into the Axis Partnership for world domination

By 1940, Adolf Hitler was well aware of the increasing tensions between the United States and Japan. The United States had embarked upon a massive increase in the size of its navy, and the Nazi leader was also aware of the risk that the United States might enter the war in Europe on Britain's side. Faced with this risk, Hitler decided to offer Japan a place in the German and Italian alliance for world domination embodied in the Rome-Berlin Axis.

For its part, Japan recognised the threat to its plans for further territorial expansion in East Asia created by the expansion of the American navy. The non-aggression pact signed by Germany and the Soviet Union in August 1939 made it essential for Japan to strengthen its ties with Germany and Italy, and to take steps to remove the Soviet menace to Manchukuo.

On 27 September 1940, Germany, Italy and Japan signed the Tripartite Pact. The agreement recognised Japan's self-assumed role in establishing a "New Order" in East Asia, and provided for mutual assistance should any one of the three powers be attacked by another country not already involved in the European conflict or the war in China. The Germans and Italians wanted the pact to convey a clear warning to the United States that it would face war with Japan if it entered the war in Europe on Britain's side.

The Americans move their Pacific Fleet from California to Hawaii

Recognising that diplomacy and economic sanctions had failed to persuade Japan to withdraw its armies from China, in 1940 the American government decided to move its Pacific Fleet from California to Pearl Harbor in the Hawaiian islands. It was designed to demonstrate the naval power available to the United States in the Pacific region, and hopefully act as a deterrent to Japanese aggression against American, British and Dutch possessions in East Asia. The action was not without significant risk, because it placed the fleet within striking distance of Japan's own powerful navy.

Admiral Yamamoto plans the destruction of the United States Pacific Fleet

Admiral Yamamoto had initially been a strong opponent of war with the United States because he knew Japan could not match its industrial strength and resources. When the alliance with Germany and Italy was signed in September 1940, and war appeared inevitable, Yamamoto fell into step with the militarists. In early 1941, Yamamoto was appointed Commander-in-Chief of Japan's Combined Fleet, and immediately took issue with the cautious plan of the Japanese Naval General Staff to limit naval operations to action against British and Dutch naval forces defending their country's territory in South-East Asia. He did not believe that the Americans would stand idly by while Japan attacked and seized British and Dutch possessions in South-East Asia. He also believed that Japan could not sustain an all out war with the United States for more than one year.

With these firm convictions, Yamamoto began planning a surprise carrier-launched air attack on the United States Pacific Fleet to coincide with Japan's move against British and Dutch possessions in South-East Asia. Yamamoto's plan for a surprise attack on the United States Pacific Fleet at Hawaii would involve a strike force which included Japan's six largest and best aircraft carriers. His task was rendered much easier by the United States government's decision to relocate its Pacific Fleet from California to Hawaii. As Yamamoto saw it, the destruction of the American's Pacific Fleet would give Japan time to seize the Philippines, Malaya, British Borneo, Burma and the Netherlands East Indies (now Indonesia), and gain access to the oil, minerals, rubber and other resources that Japan lacked. He was hopeful that, with its Pacific Fleet destroyed or crippled, the Americans would be willing to accept a peace settlement that would allow Japan to keep its new conquests in East Asia.

The Japanese Naval General Staff initially rejected Yamamoto's proposal for an attack on Pearl Harbor as being too great a gamble. Japan only had eleven aircraft carriers, and the admirals felt that Yamamoto's plan could put at risk their six best carriers. However, Yamamoto's threat to resign if his proposal was not accepted persuaded the Naval General Staff to accept it.

Yamamoto's judgment about Japan's staying power in a protracted war with the United States was correct, but he was wrong about the United States Navy's capability to intervene on behalf of Britain and the Netherlands in the Pacific in 1941. Japan had the largest and most modern navy in the western Pacific in 1941. After the American navy had been split into Atlantic and Pacific Fleets in 1941, the American naval commander at Hawaii, Admiral Kimmel, was left with a fleet of relatively old battleships, and only three aircraft carriers to pit against Japan's eleven aircraft carriers and the huge modern battleships Yamato and Musashi.

It can be fairly argued that Yamamoto's plan to launch a surprise attack on the United States Pacific Fleet at Pearl Harbor was a brilliant tactical first strike, but a major strategic error on the part of Japan to make a determined enemy of such a powerful nation without sound reasons to do so.

Hitler urges Japan to attack Britain's military base at Singapore

With the aim of encouraging the Japanese to attack Britain's colonies in Asia, and thereby diverting British military resources from the continuing struggle in North Africa, Hitler met Japanese Foreign Minister Matsuoka in Berlin in March 1941. Hitler urged the Japanese to attack Britain's military base at Singapore. The Nazi dictator made it clear to Matsuoka that the aim of this policy was to defeat Great Britain quickly and so keep the United States out of the war. The Japanese foreign minister agreed that the time was ripe for a Japanese attack on Singapore, but would not give Hitler a firm undertaking.

Despite his concern to keep the United States out of the war, Hitler then made an extraordinary blunder. To encourage Japan to attack Singapore, he gave the Japanese foreign minister an unqualified assurance that Germany would assist Japan if Japanese aggression in East Asia produced a conflict between Japan and the United States. Matsuoka did not tell Hitler that Japan's admirals were already secretly planning an attack on the United States Pacific Fleet at its Pearl Harbor base in Hawaii.

Being mindful of the need to protect Manchukuo from the Soviet Union, and so free Japan's military resources for its proposed attack on the United States Pacific Fleet and further aggression in South-East Asia, Matsuoka stopped at Moscow on his way back to Japan and signed an anti-aggression pact with the Soviet dictator Josef Stalin.