Nukes are actually terrible for defense purposes, since starting a nuclear war is a very ‘all or nothing’ affair, which means your choices are reduced to “Armageddon” or “Let the other side have their way” whenever a crisis comes up.

See: Russia and every final warning it has given over Ukraine. Since the US knows they have nothing conventionally and doesn’t buy that they’re going to end the world over a shipment of helmets or whatever it gets ignored.