It wouldn't even end all life in the involved countries. Submitting to military aggression is also morally wrong. Nuclear retaliation is a heroic sacrifice that does good to the rest of the world by discouraging future wars.
Furthermore, it's no longer medieval times, where the citizens are personally loyal to a king who is expected to be replaced by another king some time. The invention of nationalism guarantees civilian resistance, so the only way to safely rule a subjugated country in modern times is with a brutal North Korean-style regime.
Furthermore, it's no longer medieval times, where the citizens are personally loyal to a king who is expected to be replaced by another king some time. The invention of nationalism guarantees civilian resistance, so the only way to safely rule a subjugated country in modern times is with a brutal North Korean-style regime.