Japan and Germany were barely allies; Japan signed a non-aggression pact with the USSR and the Germans were happy about it because they didn't want any competition for the spoils they thought they would have. This is one of the pivotal things that led to German defeat because it freed Siberian troops to reinforce Moscow and reverse the German advance in winter 41. Hitler compounded the situation by declaring war on the US after pearl harbor freeing Roosevelt to open supply and support the allies, I think this is one of his biggest mistake. I think you exaggerate the amount of support the Nazis had in the US but there was certainly a strong neutrality movement. If anything the Nazis admired America and manifest destiny and tried to emulate it with lebensraum.