One scenario is that Iran unloads a fleet of conventional missiles on Tel Aviv. Netanyahu decides one on Tehran will stop all that. Five silos open up. Trump goes crazy. SecDef and his Russian counterpart are drunk. The U.S. opens its doors. At the first launch signature, Russia launches on warning against Israel and Ukraine. Why not? The US follows. Within a few minutes, the East-West confrontation starts. Communications fail. Submarines finish the job.
In my spitball model, no one is quite rational. But what rational person would choose to end life as we know it?
It's rational to threaten retaliation. It is also rational not to retaliate in the moment . Life may continue.
Putin is very much in a weak position. The Russian economy is weaker than generally assumed. Ukraine, faced with an existential threat, is very effective. Putin, like Trump, could do something very stupid.