More concretely, if Russia is allowed ANY success in Ukraine, it puts the nail in the coffin of nuclear non-proliferation. If the only thing the world does is bow to anyone who can hold it hostage with a nuclear threat, the only defense is your own nukes. If you want to avoid countries fighting nuclear war, you are better off fucking over Russia right now, and understanding that their nuclear talk is all bluff (for now) rather than wait until hundreds of tiny and unstable countries have nukes that they want to fire at each other.

If the West defends Ukraine from a nuclear armed nation, then we can convincingly tell the rest of the world "You don't need nukes, so don't build them".