America was alarmed. With important strategic and economic interests in Asia, the U.S. increased military and financial aid to China and placed an embargo on Japan. Poor in natural resources, Japan needed American oil and other raw materials. By the end of July, 1941, all oil shipments were cut off. Government officials in Tokyo vowed to get their own oil - by conquering all of Southeast Asia.

Former allies in World War I, both countries had aligned themselves with opposite sides as war broke out in Europe. America was giving aid to Great Britain whose empire stretched throughout Asia. In 1940, Japan signed an agreement (the Tripartite Pact) with Germany and Italy. Japan’s oil supply increased the range of Nazi war ships.

During 1941 diplomats from both countries tried to negotiate their differences. But the Imperial Government hedged its bets. Its leaders wanted to continue their expansionist policy. Admiral Isoroku Yamamoto wrote a letter on January 7, 1941 suggesting a plan that would immobilize the one perceived obstacle standing between Japan and her intended conquests: the American fleet at Pearl Harbor.