If that were true, there would be no fighting at all, because that necessarily implies that Russia would have withdrawn back behind its own borders. That is all Russia has to do to stop the bloodletting - return to its own borders.
Russia would be fighting on someone else's territory and someone else's terms. And since Ukraine boyscouts can do this to topline Russian tanks with U.S. javelins and artillery,
there can be little doubt just how badly Russia would get its a$$ handed to it if NATO troops were using those weapons.