“Substantial US air and ground forces are in Poland: does anyone believe that they’ll wait for Congress to declare war before engaging if the balloon goes up?”
The Biden regime isn’t worried about Congress, they assume they can get the votes there. It’s the EU (NATO) they want to force into this war via treaty obligations.
I don't they're trying to force anyone into anything: they offered Zelensky the rooftop helicopter ride out. The expected Ukraine to collapse almost immediately, and even afterwards hedged their bets by only doling out military aid in small chunks. Only the Germans have been dragging their feet, mainly because of the SPD's close ties to Putin. By having done so little, Germany now plays almost no role in determining the outcome of the war despite the fact that they will suffer the most from the loss of Russian gas.
Ukrainian resistance turned out to be far stronger that they or Putin expected, which is now leading to Putin trying to up the ante and coerce Ukraine and NATO to accept his annexations. That won't work.