Background to Pearl Harbor
As 1941 progressed war between Japan and the United States became all but inevitable. Japanese expansion had begun with its invasion of Manchuria. Japan became embroiled in a war with China that it could not seem to win. The United States while remaining officially neutral clearly sympathized with the Chinese.
When Germany, who the Japanese were allied with attacked the Soviet Union, the Japanese made the strategic decision to move south and seize Indo-China. This action resulted in the freezing of Japanese assets in the United States together with a trade embargo. This all but made war with Japan all but certain.
While war was inevitable the timing of it was not. The United States, which was preoccupied by Nazis Germany in Europe, and was in the midst of a massive rearmament, wanted to do all it could to delay the war. The Japanese however, were looking at the same timetable and knew that by late 1942 a steady stream of large naval ships would be joining the US fleet. The US embargo was also taking a toll on the Japanese economy. Thus the Japanese knew they needed to take dramatic action.