Germany declared war on the U.S. shortly after Pearl Harbor.
Yes, how convenient for FDR. After siting anxiously on the sidelines, itching to get in (as he actually should have been to come to the defense of allies), Germany acted the only way it could after Japan's "surprise attack" on the US.
But, in keeping with the intent of the thread, it was the Paulian-like isolationism that actually compounded and lengthened an already long and bloody war. FDR was right to want to get into the war earlier on. But, it was the old-style isolationism of the GOP that kept him from doing so. That was a mistake that cost tens of thousands of US and allied lives, not to mention the lives of perhaps millions of Jews, amongst others.