There were two leaders of Japan during World War II: Hirohito and Hideki Tojo. Hirohito was emperor of Japan, while Hideki Tojo was Prime Minister, a general of the Imperial Japanese Army and the leader of the Imperial Rule Assistance Association.
After the war, in order to avoid persecution, Hirohito portrayed himself as a powerless constitutional monarch. However, scholars believe he actually played an active role in the war effort.
After Japan surrendered in 1945, he became a figurehead with no political power. As Prime Minister, Tojo was directly responsible for the attack on Pearl Harbor, which initiated war between Japan and the United States. Tojo was eventually arrested and sentenced to death for Japanese war crimes; he was hanged on December 23, 1948.