Answer:
Japan invaded China and many Americans were upset by this and protested. Japanese leaders requested a meeting with FDR, and he told them to pull out of Japan. FDR also did not want Japan to expand so he cut off all trade with them. The Japanese wanted to take over half the worlds population and to do so they needed to take out American naval forces in the Pacific.
...er, I assume previous poster meant that FDR wanted Japan to pull out of CHINA...
Japan began their conflict with the U.S. when the U.S. cut off trade to them for their refusal to cease their invasion of china. With only about nine months worth of supplies left to fuel their war effort, the Japanese demanded control of the Philippines to get their minerals, fuels, rubber, etc. When the U.S. refused, American analysts assumed they would take it by force and stepped up their pacific fleet. What they did not realize was that carriers with roughly 240 torpedo bombers and fighters was creeping ever so closer to the Hawaiian islands. On December 7, 1941 a radar operator noticed a massive concentration of enemy aircraft but it was too late. The aftermath is obvious and the next day the Philippines fell to Japanese invasion, launching a dormant giant into the second great war.
So basically attacked because supplies were limited unless the Japanese took pacific islands at their leisure which the united states was fully against.