The Russians agreed to the borders when the USSR broke up and in subsequent agreements guaranteeing Ukrainian territorial integrity, in one of those agreements Ukraine gave up its nukes and America and the UK gave security assurances too. According to the UN charter the Russians broke the law and the international system of peace and prosperity. Russia has no claim on Crimea or any of the other republics they occupy, soon it will be the same rationalization when they try to occupy and absorb Belarus. The Russians even lost the referendum in Crimea, it was 57% there to join Ukraine and by the time the Ukrainians are done starving them out, many of the Russians who moved there will leave and are leaving in droves now. Russia invaded Crimea in 2014 the same way the invaded Georgia before and the same way they took over other places and the same way they are threating to take over Moldova. It's imperialism plain and simple, the people living in a place make the call and they did when the USSR broke up, not Putin and a gang of criminals in Moscow.
We will destroy their army and ability to project force, they can only operate about 50km from their wide gauge railheads anyway and this rail gauge is used in their former empire. By destroying critical rail bridges on the half dozen rail routes leading into Ukraine up to 100km inside Russia, they can be cut off and trapped in Russia. Rail tracks can be rapidly repaired, rail bridges take a lot of time, especially for Russia and especially when the repair crews and equipment are destroyed periodically by drone and missile attacks. The Russian military and economy are heavily dependent on the rail network, and they also have weak rail links crossing many bridges to the east of their country, Siberia and the eastern pacific coast. IMHO rail bridges are their weak link and destroying a few critical ones would see the Russians in a lot of trouble, many are in isolated locations in the middle of nowhere and very hard to defend.