Control of the Japanese government in the 1920s and 1930s was increasingly in the hands of the military and expansionists. The Japanese developed the idea of the “Greater East Asia Co-Prosperity Sphere,” the concept of a Japanese sphere of influence encompassing Southeast Asia (Indochina), China, the Philippines, and the southern Pacific Islands. They were determined to exert their control over that area. The Japanese viewed the United States as a major obstacle to their expansionist desires.
The first step in the Japanese expansion plans happened when they seized Manchuria in 1931. Japan launched a full-scale invasion of China in 1937. Within a year, Japan controlled all but China’s western interior and threatened all of Asia and the Pacific. On September 27, 1940, Japan signed the Tripartite Pact with Germany and Italy, thus entering the military alliance known as the “Axis.” Then, in July 1941, they turned their attention to Southeast Asia: the French Indochinese colonies of Vietnam, Cambodia and Laos along with the British possessions, Burma and India.
The United States was concerned that its economic interests in the Pacific would be negatively impacted by Japanese expansionism. In reaction to Japan’s continued aggression, in mid-1941, President Roosevelt froze Japanese assets and cut-off oil supplies and scrap metal in an effort to slow down the Japanese. The United States could do little more at the time because of continued isolationist sentiment and lack of military preparedness.
The Japanese knew that the United States could not sit back and let them take over the Pacific with impunity. They assumed that, ultimately, there would be a military confrontation. A surprise attack on Pearl Harbor had long been part of Japanese war planning. The military, which strongly influenced the direction of Japanese foreign policy, argued that the U.S. embargo made it imperative that the attack be carried out as soon as possible before the embargo had a negative effect on the Japanese war machine and the United States had the opportunity to re-arm. (TFC, vol. V, p. 66.) The Japanese military leaders reached a compromise with the civilian leaders, who wanted to avoid war. If the diplomats could not negotiate a resolution of the conflicting interests with the U.S. by the end of November 1941, then the military would be given the green light to commence operations. (Id.)
The United States broke the Japanese diplomatic code in January 1941, so from that time forward, America was aware of the debate that was on-going in Japan. (Id.) The U.S. had intercepted the following messages in November, 1941:
November 4 – “Relations between Japan and the US have reached the edge. This is our last effort”
November 22 – “We will wait until Nov. 29. After that things are automatically going to happen”
November 28 – “In two or three days negotiations will be de facto ruptured.” (Id. at 68)
Other intercepted messages gave greater credence to the likelihood of a Japanese attack. (Id. at pp. 68, 70.) Unbelievably, although other possible targets were on the highest state of readiness, Pearl Harbor was “on its loosest alert – Condition Three, meaning that but 25 percent of its antiaircraft guns were manned.” (Id.)