NATO exists primarily to counter Russia and the Soviet Union. The European defense posture assumes America is an ally. Previously America has cut off intelligence sharing with Ukraine and that includes targeting data for missiles. America has also put restrictions on what Europe can do with weapons they've bought from America.

Then there's the fact that Biden willfully invited this invasion by suggesting Ukraine join NATO (with Biden himself previously saying publicly that that would be a red line for Putin) while also repeatedly stating America won't defend Ukraine if Russia does invade.