What if Hitler doesn't declare war on the United States after Pearl Harbor After Perl Harbor the US declares war on the Empire of Japan and doesn't seem to have any interest in declaring war on Germany. This is Germany's best case scenario, The aid to Europe will diminish or disappear as the US focuses all its resources on a War in the pacific. Germany can also decrease their worry about the US getting involved in their European theater, which was one of Hitler's fears and why unrestricted submarine warfare against neutral nations was never authorized even though it allowed vital supplies to get to the UK. 

We may never understand why Germany went against its doctrine of never declare war even though their treaty with Japan didn't require it but how does the war change if the US only cares about retaliation against Japan focuses all their efforts to the pacific.