Answer:
Japan was looking for oil.
Tojo wanted to keep the US out of the war for long enough to give the Japanese an advantage.
Tojo was an advocate of war against the US, while Roosevelt rejected traditional US isolationism and switched towards a more active foreign policy.
Explanation:
In the years leading to World War II (1939-1945), Japan and the United States had become the major powers in the Pacific, and despite their clashing interests, they remained trade partners and were on generally good terms. However, by the early 1940s, their relationship rapidly deteriorated. The US didn't approve of Japan's expansionism in Eastern Asia and continued shipping weapons and other supplies to China, which was partially occupied by the Japanese. On July 1941, the US suspended oil exports to Japan. This threatened the whole Japanese economy, which had become dependent on oil from the US. Negotiations bogged down as the US demanded that Japan should withdraw from China and refrain from attacking any of its neighbours. The Japanese, led by Prime Minister Hideki Tojo, saw no way out of this dilemma but to seize the rich oil fields of Southeast Asia, especially in the Dutch East Indies.
However, the United States had openly committed itself to defending the Southeast Asian colonies held by the Western powers, including the East Indies and the Philippines. Tojo estimated that by causing enough damage to the base in Pearl Harbor, in Hawaii, the US would stay out of the war for long enough to give the Japanese the time they needed to conquer Southeast Asia before any significant opposition could be mounted. The Japanese moved forward with their plan, and attacked Pearl Harbor on December 7, 1941, prompting the US to enter World War II.
In the leadup to this event, Prime Minister Tojo and US president Franklin D. Roosevelt played major roles. Tojo was an advocate of war against the United States, and tried his hardest to steer opinion within the Imperial ruling circle towards war, as many were still hesitating. Meanwhile, Roosevelt steered public opinion in the US away from isolationism and towards a more active foreign policy, framing WWII as a struggle to defend freedom and democracy in the world.
