Suppose Germany gives no assurances to Japan it will join Japan in a war with the USA? Japan attacks the USA in 1941 anyhow. Germany does not.
Germany decides the inevitable showdown can wait until when and if it wins its present war? Unless the USA declares war beforehand.
If FDR wants to "give" Germany a role at Pearl Harbor, can he be sure he can sell that?
What does all that do to or for the America Firsters and other isolationists?
What will opinion amongst those who could do anything about it, and, in the case of elected officials, those that vote for them, be about continuing to send resources to Germany's enemies that could instead be used aginst Japan?