After the war, the United States occupied Japan and controlled everything that happened there. In December 1941, it looked as though Germany might defeat the allied forces in Europe. America would not allow Hitler to control a large portion of the world and would soon enter the war. Japan was allied with Germany and would be forced to fight America. Also, if Hitler did win, Japan wanted to seize China and the rest of East Asia.
Japan has few natural resources of their own, but they had a great army and navy.
They seized northeast China and much of Southeast Asia prior to the war. The United States seemed likely to attack the Japanese in order to support the other Asian nations. The Japanese felt that war with the United States was coming anyway, so a sneak attack would give them a head start on the Americans. Japan destroyed the U.S. naval fleet in the Pacific Ocean, but they unified the American people. Americans had not felt strongly about a war fought far away and had little interest in joining the conflict.