Why are you picking the oblique and obviously implausible case of Putin trying to "scare" the West by detonating a device on his own territory (and as far away from his adversaries as possible)? What people are worried about is a low-yield device over the Black Sea or the North Sea, say.

The "pathway" is that any deployment of a device will at the very least trigger a suite of necessary countermeasures on both sides (moving assets around and so forth), greatly increasing the risk of misperceptions and panic reactions.

I don't understand what trick. Russia directly attacking NATO?

The "trick", by which I mean outcome simply, of pushing the regime into an (acutely) fatalistic mindset. The reaction doesn't have to be a direct attack on NATO. It could be some other form of escalation (of the sort they might have ruled out otherwise). It doesn't even have to be an action taken by Russia. It could a matter of China looking at the situation and thinking, in terms of the move they've always been threatening to make, "now or never".

I'd rather not delve into scenarios; I'm just advocating that we observe the cautionary principle here. We agree that the risk is overstated, and that the aggressor's threats are mostly bluster. But it's definitely not negligible risk, in my view; probably on the order of 3-5 percent per year. As high as at any time since the Cuban missile crisis, in any case.