MAD is the only way. The current powers are just too weak to enforce it. Ukraine shutdown its nuclear arsenal because of russia and the us. The US should, if it had any balls or was worthy of trust, declare that any nuclear attack on ukraine will be seen as a war declaration to the US.
The situation in Ukraine only shows the world that non proliferation is a stupid gamble.
https://en.m.wikipedia.org/wiki/Ukraine_and_weapons_of_mass_...