Why was japan at war with the US?

Answer 1

Because Japan coveted the oil-rich Dutch East Indies (modern Indonesia) and wanted to knock out the U.S. Pacific Fleet (at Pearl Harbor, Hawaii) so that it wouldn't be able to present a threat to their operations in the East Indies. Japan also invaded the Philippines (then a U.S. colony) so that the American forces based there wouldn't be a threat to their sea lanes between the East Indies and Japan, the sea lanes on which its oil tankers would have to travel to bring the oil to the Japanese home islands.

Basically Japan's government didn't feel comfortable with leaving a potentially hostile American bastion (the Philippines) astride its sea lanes between the East Indies and Japan even if the U.S. didn't get involved in the war when Japan would move against the East Indies, which Japan presumed anyway that the U.S. would get involved. Japan's attacks against U.S. forces in Hawaii and the Philippines (and Guam, and Wake Island) were intended to prevent American interference with Japan's acquisition of the East Indies.

Japan's government felt pressed for time because in the summer of 1941 the U.S. had cut off American oil shipments to Japan (because of Japan's move into French Indochina) and though Japan had some oil stored away just in case it wouldn't be enough to last for more than a few months so unless the U.S. agreed to restart oil shipments soon Japan's government felt it had no choice but to seize the Dutch East Indies to ensure it had enough oil to fuel its country and its war effort in China. The U.S. government had no intention of ending the oil embargo and when this became clear Japan went forward with its preemptive strike against Pearl Harbor and operations against the Philippines, Wake Island and Guam. Japan also invaded British Malaya (modern Malaysia) because it saw the British as another obstacle to the expansion of its empire.