MAD applies if all sides are actually at risk of being destroyed. A close historical precedent for the current situation is the US against Japan in which the n

MAD applies if all sides are actually at risk of being destroyed. A close historical precedent for the current situation is the US against Japan in which the nuclear power didn't have a reason to destroy its enemy (indeed, worked to rebuild it immediately after), and the enemy couldn't retaliate on par. So there was no escalation and instead a quick capitulation. If Russia uses tactical nukes against Ukraine, it matters how it uses them. Presumably it would like to use them as the US did, so it can take Ukraine without destroying it and put a faster end to the war than otherwise possible. The US can then concede, escalate the proxy war by giving Ukraine tactical nukes, or strike Russia directly. The latter still looks mostly like MAD to me. A nuclear proxy war limited to tactical nukes is.. unfortunately all too easy to imagine, with some precedent in the Turkish/Cuban missile crisis. That could then escalate until Russia dominates with its strategic nukes, but presumably Ukraine would capitulate before that. The lesson of high cost to Russia would also be there for future reference.