I'm genuinely curious at what point a bully threatening you with MAD can be stopped if you value the existence of your own civilization? Say Russian conquers all of Ukraine and moves troops to the Polish border and parks some tanks a bit inside. What then? Strongly worded essays? If Russia isn't raining hell on Poland, then NATO is forced to .. apply sanctions, make threats, but take no action since the escalation an…
Your analysis seems rational in this moment. But: the policy and personality of a democracy is liable change rapidly and radically in a way no single actor ever would or could. Autocrats often do not understand this, and make the same assumptions you have.
The scenario you describe would raise such apocalyptic outrage in America and Europe that war without thought of consequence would likely ensue.
You simply cannot extrapolate future behavior of complex democracies from current behavior as you would an individual. The concern you laid out in your post is relevant: once enough citizens arrive at a similar opinion, policy can rapidly shift.