I think your 5% figure of KABOOM given retaliation fails to condition on kaboom.
I would estimate an 8% chance of kaboom (broadly following prediction markets, going by a 10% chance of the order to nuke Ukraine and an 80% chance of it actually happening) and an 80% chance of retaliation. For KABOOM, either the West or Russia would drop the first strategic nuke. For the West, the probability is somewhere around 0.1% given kaboom. For Russia, while the probability of KABOOM given escalation is probably less than 5% (more like 1%, I’d guess), kaboom has happened, and so the probability of Russia dropping a strategic nuke is closer to 12.5%. And so, the final probability of KABOOM is closer to 0.9% than 0.25%.
I think your 5% figure of KABOOM given retaliation fails to condition on kaboom.
I would estimate an 8% chance of kaboom (broadly following prediction markets, going by a 10% chance of the order to nuke Ukraine and an 80% chance of it actually happening) and an 80% chance of retaliation. For KABOOM, either the West or Russia would drop the first strategic nuke. For the West, the probability is somewhere around 0.1% given kaboom. For Russia, while the probability of KABOOM given escalation is probably less than 5% (more like 1%, I’d guess), kaboom has happened, and so the probability of Russia dropping a strategic nuke is closer to 12.5%. And so, the final probability of KABOOM is closer to 0.9% than 0.25%.