The US has allegedly said they will retaliate with nuclear strikes on Russia if Russia uses nuclear strikes on Ukraine.
Barring an attack on the US itself, the US under the current regime will never attack Russia. Whatever the kompromat happens to be, the President is completely bound by it.
That'll never happen.
Honestly, I thought part of MAD was how, once a nuclear missile was launched, it would be better for other nuclear states to decimate the country of origin than to wait and figure out where it would hit.