There are three points of having nukes:
1. Deter other countries with nukes from using them against you, or your military ally.
2. Prevent total annihilation in the war. You can lose the war, but not too much.
3. Burn the world to ashes. Very few countries can do it. It effectively forces the whole world to make sure that this scenario does not happen. So you can be sure that scenario where Ukraine conquers Russia and completely destroys it - will be prevented by the very Ukraine supporters. They don't want to live in the nuclear post-apocalypse, because there are scenarios where Russia fires every single nuclear missile on every major city on the Earth. As Putin framed it: We will go to heaven as martyrs, and they will simply drop dead.
America lost several wars, recently they lost Afghanistan war and right now they're losing Iran war. They won't invoke nukes to overturn the table, they'll accept the lose.
