If Hitler declares war after Japan turns back

How would Roosevelt react if Hitler declared war after the Japanese force turned back?

The declaration would simplify Roosevelt's position and allow the United States to focus most combat power on Europe.

If Germany declared war while Japan denied involvement and withdrew, Roosevelt would gain a clear legal and political path into the European conflict. Tokyo would probably claim no knowledge of Hitler's decision and continue stepping back rather than join him. The Far East would still require garrisons, but not the forces consumed by an active Pacific war. The United States could therefore commit far more combat power against Germany and probably shorten the European war.