Actually, the fact that Japan was an Axis member had nothing to do with the US entry into the war against Germany, except for the fact that Germany declared war on the US. The private papers of Franklin Roosevelt and Harry Hopkins (his trusted advisor), that were released in the 1990's after the 50 year limitation on Top Secret documents Act that kept them secret for so long, verified what many historians and Roosevelt critics of the time had accused him of . . . that the US would NEVER have entered the war unless war was declared on them or (Pearl Harbor) they were physically attacked. Roosevelt's papers register his relief that on 12/10/41 Hitler declared war, because otherwise the US would have had to fight only Japan. Those same papers verified that the private, personal agreement made between Churchill and Roosevelt as to the intended conduct of the war was made in secret by both from even their own governments and carried absolutely NO WEIGHT until after Hitler declared war. There were those in the US Congress with great power who could and would have (documented in their own papers) gone as far as to start impeachment proceedings based on Constitutional grounds of the Powers of War clause subversion had he tried to initiate a declaration of war on Germany. We would have done what the USSR did when they refused to attack Japan just because Japan joined the Axis, but never attacked them. Most people don't know that and the histories written by most historians gloss over that fact to make it appear that entry into the war in Europe was automatic when Japan attacked. Roosevelt's own writings say that that was NOT so and that he was still worried about how to get an excuse, politically, to become actively involved in Europe. It was against US political policy at the time to declare war on Japan's Axis Allies, because we did not recognize the alliance as active unless a participant declared war on us or committed an aggressive act. He gambled, with good odds, that the psychologists and political international advisors' predictions that Hitler would do it himself; which he did. But, if Hitler had not backed up his alliance with that declaration, the US would have ONLY fought the Japanese. . . at least until the Germans had sunk another US ship going to England with Lend-Lease materials.
PS - There are still some papers that can't, by law, released to the public until 100 years from the date they were sealed (i.e. 2040 to 2050) from Roosevelt, Truman, and others' papers that might have some equally surprising revelations about the war and its conduct. Much like those released in the 1960's from the Lincoln/Civil War Era. Unfortunately, most western history, once written and accepted for long periods, will never be revised. The histories are written by the victors to tell the stories the way they want them to.