"Joshua Ball explained that 'if Russia were subjected to a major non-nuclear assault that exceeded its capacity for conventional defense, it would 'de-escalate' the conflict by launching a limited — or tactical — nuclear strike.'
It might go something like this, per Ball: after a few more months of quagmire, Putin orders a tactical nuclear barrage to blast holes in Ukraine's battlelines. At the same time, he sends a message to NATO: 'Look, neither of us wants a strategic nuclear exchange. Withdraw your support for Ukraine, and this can all be over. Your move, Biden'."
From Biden worried that Putin 'doesn't have a way out' of Ukraine by Grayson Quay in "The Week."