There are not enough Ukrainians to conquer Russia, and Russia would simply wipe them off the face of the Earth if they became too annoying.
Russia will decide what happens to the Ukraine now. The only way to stop Russia would risk killing everyone.
I am against intervening in a EU conflict but... there is no risk of a nuclear exchange between anyone even in a hot war scenario. If nuclear weapons were so effective, then why didn’t Putin just drop one on Kiev and call for surrender? Why is he killing his own troops now when one bomb would have done the trick?