So why did Japan attack Pearl Harbor? Because the US fleet is the only threat to Japan While the attack seems suicidal at first, in hindsight, British, France, and the Netherlands Eastern colonies were largely defenseless after June 1941, 6 months before the attack on 17th December 1941. Japan and Germany could have worked together to take down Soviet Russia after Japan take over these colonies, all while avoiding the US. Russia, however, is a hardy opponent. The USA was not expected to join the war at all. Japan make one mistake, with their lack of intelligence, they are convinced that the US naval fleet is still weaker and less modern than the ones Japan owned (Japan was considered to own the best air fleet). USA did not interfere at all when the UK gets attacked 26 months earlier on September 1939. So Japan is overconfident that their attack would not receive too much backlash and is indeed advantageous to their goal. So they attacked, expecting only a negotiation from the US, instead the US entry into the war and two bombs that devastate their country.