Question

What was Japan's motive behind dropping the bomb on Pearl Harbor?

Answer 1

Japan wanted to deplete the U.S. Navy's forces, so they wouldn't have such a hold on the Pacific. The reason why they attacked was kind of out of spite for the U.S. because Japan and  China were fighting a war and the U.S. supported China and refused to give Japan any oil and placed a trade embargo on them.

Answer 2

By making a premptive  attack the Japanese hoped to cripple the American Navy.They believed that they could not win the War without the element of surprise. I'd say this backfired on them, since the the US  not only entered the War but also dropped a bomb on Japan