The Nazis did sink the USS Reuben James & no war declaration. Let's say however that a large, lit/identified & marked US passenger liner is sunk with 100's of dead Americans, and a US destroyer coming to the rescue also sunk - in the western Atlantic, not near the UK. That just might do it. So US enters the war in June 1941.
May get some shift of US fleet to Atlantic, and even without that this helps the RN big time. Still get the disaster of no convoys/blackouts along the east coast like OTL "Operation Drumbeat" by the Germans, but that gets solved like OTL, maybe even quicker. I expect USAAF units go to Britain, primarily fighters & pace of lend-lease stepped up.
The alliance Japan had with Germany was defensive & I doubt they would declare war in this case - Germany was not "attacked". Of course with the USA at war, even if vs Germany, there is no way a sneak attack on Pearl harbor happens.
Big question is, assuming declaration before Barbarossa, does Hitler still go?