We can find two most decisive factors. The first was in the changing European situation after the outbreak of the German-Soviet war. The United States was able to have enough self-confidence to fight against the Axis powers, as the Soviet Union was a superpower in the world. The second was in the Southeast Asian question. The United States had fears that Japan would advance into Southeast Asia.
In short, the true causes of the outbreak of the Japanese-American war were these two factors of the American-Soviet rapprochement after the outbreak of the German-Soviet war and Japan's advance southward.
Ike, N. (ed.), Japan's Decision for War; Records of the 1941 Policy Con- ferences, Stanford, Calif., 1967, pp. 209-210.
Schroeder, P.W., The Axis Alliance and Javanese-American Relations 1941 (Ithaca, New York, 1958, Fourth printing 1971) p. 203.
United States Army in World War II, China-Burma-India Theater, Stil- well's Mission to China (Office of the Chief of Military History, Depart- ment of the Army, Washington, D.C., 1953) p. 12.
Papers relating to the Foreign Relations of the United States, 1941, IV, pp. 124-125.
Kennan, G.F., American Diplomacy 1900-1950 (New York, 1951) p. 93.
Foreign Relations, Japan, 1931-1941, II, pp. 281-282.
United States Army in World War II, The War Department, Vol.4, pt.l:
Prewar Plans and Preparations (Office of the Chief of Military History, Department of the Army, Washington, D.C., 1950) p. 80.
Feis, H., The Road to Pearl Harbor (Princeton, N.J., 1950) p. 236.
Iriye, A., Across the Pacific: An Inner History of American-East Asian
Relations (New York) 1967, p. 201.
Spotswood, R.,"Japan's Southward Advance as an Issue in Japanese- American Relations, 1940-1941" (Ph. D. dissertation, University of Wash- ington, 1974) p. 9.