During World War II (when the US was neutral), American firms made plans for the British, and then transported them to the Canadian border, and British (which back then included Canadian) workers walked them over the border. If they had been flown from the US to British controlled territory it would have been an act of war, and the US was trying to be neutral. This might be a problem now since even if the planes could be driven (using their wheels, not flying) over the border, the moment the entered Ukraine they could be attacked.
At this point, Putin is failing miserably at his objectives, and might feel the only way to salvage the situation is to use nuclear weapons. The question is whether Biden (and the American people) would be willing to fight a nuclear war, with fatalities perhaps in the billions, in response to Russia blowing up Warsaw or Bucharest or Vilna. As it is, most Democrats believe that America’s days of greatness lay in the past, and remembering Vietnam and more recently Afghanistan, don’t believe there is any will of the American people to go to war. Trump’s views are not all that different that Biden’s, but whereas Trump is challenged within his party by the “old fashioned” Republicans of the Reaganesque style, the “Progressive” Democrats are super-doves and Biden feels he can’t risk antagonizing them.