What if Biden takes a U turn and US military enters Ukraine as peace keepers ? Putin didn’t kept his promise of not invading Ukraine so US can also bolster its troop at Poland and send them in Ukraine if invited by Ukraine. Since a Hypothetical US/Russia clash might happen inside Ukraine so Russia won’t have any moral reasoning to use Nukes either. What are a chances of this scenerio taking place ?
Next to 0.
Firstly, Biden has repeatedly stated that he is not sending troops to Ukraine. If he breaks his promise there will be internal pressure on him. Keep in mind that the mid-term elections are coming.
Secondly, if you are sending airborne units into Ukraine, the same thing that happened to Russian airborne units will happen to the Americans. Too many Russian heavy equipment on the ground. Air control cannot be immediately secured. It will be a bloody affair and imagine how the public will react seeing American troops captured by Russian forces.
Also, moral reasoning cannot stop Russia from using nukes. If Russia feels that it can control the intensity of nuclear warfare (I know the US does), it may still choose to use tactical nuclear weapons. Afterall, it is Ukraine that gets contaminated by fallout.