Japan needed resources to feed its war effort in China. The US had stopped trading with them because of this war, having embargoed scrap steel and oil. In addition, the US was in the way of Japan and its desired realm of expansion-to solve their resource issue-namely, the Dutch East Indies. They were also not very happy with Japan's expansion and had set severe embargoes on Japan in 1940. So they had to neutralize the US assets in the area to prevent them from interfering with their conquest of British Malaya and the Dutch colonies.