Why the USA haven't intervene like in Yougoslavia, Irak, Syria, Afghanistan, Lybia if they really pretend to be an ally of Ukraine ? 
Why would only Russia be allowed to attack with his allies and not Ukraine ?
If Biden had said that he would intervene in case of a russian attack never would have Putin dare invade. And for this I recognise Trump was true in his says that he wouldn't have allowed Russia to enter Ukraine.