Why didnt US declare war on Germany during World War 2?

It has been US policy (tradition) NOT to declare war on any nation without cause. Germany did nothing to the US, worth declaring war over (they had engaged some of our "Flush Deck-four piper destroyers" that were performing escort duties in the Atlantic Ocean). It was the "Tripartite Pact" that allowed the US to enter war with Germany; When Japan attacked, Germany tried to fulfill their end of the bargain...hoping in return, Japan might attack RUSSIA (Soviet Union); creating a "Second Front" for the Soviet Union then fighting the embattled Germans.