The Japanese leadership watched the events in Europe closely in 1939 and 1940 and found in Germany's rapid victories the encouragement it needed to expand its control into Southeast Asia. The expansion was necessary to meet Japan's desperate need for natural resources such as petroleum and rubber to help carry on its military efforts against China and elsewhere.

Its reliance on supplies from other countries was a major problem for Japan. As a result of a commercial treaty signed in 1911, a high percentage of Japan's resources came from the United States, including oil (66 percent), aviation fuel (100 percent), metal scrap (90 percent), and copper (91 percent). But in July 1939, President Roosevelt, angered by Japan's growing militarism, threatened to terminate the treaty and sever this essential source of supplies. Realizing that it could no longer rely on the United States for resources, Japan began to hungrily eye its neighbors.

The descent into war in the Pacific began to accelerate. When France, Belgium, Luxembourg, and the Netherlands finally fell to the Germans, the government of Prime Minister Fumimaro Konoe announced Japan's plan to create a self-sufficient economic and political coalition under Japanese leadership to be known as the Greater East Asia Co-Prosperity Sphere, which would dramatically change the balance of power in the Pacific because the defeated European nations had ruled much of southeastern Asia as colonies.

Then, in September 1940, the Konoe cabinet joined the Axis by signing the Tripartite Pact. In so doing, Japan received permission from Germany to occupy French Indochina, which had been a source of supplies for China. In Indochina, Japan would be able to establish bases it could use to conquer nearby regions rich in the natural resources it so desperately needed. Of special interest to the Japanese were the Pacific colonial territories of the Netherlands and Great Britain, particularly the oil-rich Dutch East Indies.

Meanwhile, relations between Japan and the United States continued to worsen. After a series of shakeups, the Japanese cabinet hardened its expansionist policies. The message was clear: Japan needed land and resources and was more than willing to take them by force. On April 13, 1941, Japan and the Soviet Union signed a neutrality pact that greatly benefited both countries. The Soviet Union agreed to respect the borders of the puppet state of Manchukuo, and Japan pledged to do the same in Mongolia. The pact gave Japan important protection on its Soviet flank and opened the door for war against the United States and the European colonial powers.

Japanese militarists wasted little time. In July 1941, after removing the remaining moderates, the military-dominated Japanese cabinet ordered an invasion of the rest of Indochina and demanded more conscripts. U.S. President Roosevelt responded to Japan's increasingly warlike attitude by freezing all Japanese assets in the United States and cutting off oil exports to Japan. Prime Minister Fumimaro Konoe struggled for a diplomatic solution, but the armed forces favored a more militant policy and urged at a conference in October that the nation prepare itself for war.

Konoe resigned as prime minister and was replaced by General Hideki Tojo, Japan's minister of war. Tojo, a career military man, drew a line in the sand when he demanded that the United States cease sending aid to China, accept the Japanese conquest of Indochina, resume normal trade relations, and not reinforce American bases in the Far East.

Negotiations between Japan, represented by Ambassador Kichisaburo Nomura, and the United States, represented by Secretary of State Cordell Hull, quickly hit an impasse. On November 26, the United States again demanded that Japan withdraw from China and Indochina if negotiations were to continue, but Japan wasn't about to give back what it had already taken. Considering Hull's demand an ultimatum and thus seeing no need for further talks, Tojo ordered to sea a Japanese task force that had been assembled secretly off the island of Etorofu. Its target: the U.S. naval base at Pearl Harbor.