Nukes only protect from catastrophe (if others think that your government is sane) because they're essentially a "fuck you, we all lose" button. If you reply to

Nukes only protect from catastrophe (if others think that your government is sane) because they're essentially a "fuck you, we all lose" button. If you reply to something with nukes, you get nuked too. What this means in practice is that something like "Russia assassinated the leaders of France + Germany and conquered Poland in the ensuing chaos" isn't enough of a justification for France to use nukes. That's because the payoff matrix looks something like this: peace * conventional war * nuclear war France 10 * 5 * -10,000,000 Russia 5 * 10 * -10,000,000 Sane governments don't choose to move their payoff from 5 to -10,000,000. Now, there are exceptions here! One is insane government leaders. It takes a special kind of person to light the world on fire because someone took their toy, but... no one takes that guy's toy. And so since they can credibly rule out column #2 above, they never leave column #1. The other exception then is when a sane government credibly precommits to the use of nuclear weapons in some situation. But it's rather hard for a liberal democracy to do this, and if France said that they'd kill Russia with nukes (and thus indirectly themselves, Poland, and everyone else) in response to Poland being invaded - well, I don't think I'd believe in their ability to follow through.