So, Ukraine's not in NATO but NATO would feel threatened by Russian soldiers there, why? That if Russian takes Ukraine then it will move on NATO? I'm not going to lose sleep over that idea.
How did Iran feel when 200,000 US soldiers were in Iraq on its one side and in Afghanistan on its other side.
On the other hand, if NATO wants to send in troops, then NATO will send in troops, American and otherwise.
Not that anyone's asked, but I think it's silly to send in troops. Hack into some industrial or government computer and disable their trains or knock out the heat in Moscow.