He knows it would be strategically impossible for the US or Nato to respond in kind to a nuclear attack on Ukraine. It can be rational for him to climb the escalation ladder if he knows his enemy will have to back down before he does because they have less to gain and more to lose.
I don’t like this situation at all, and I understand the point about rolling over to threats. Unfortunately, this is just how nuclear game theory works whether we like it or not. You can’t win in a conflict under MAD against an opponent who has more at stake than you.
And of course there can be an advantage for Russia in using nukes. They could destroy Ukraine's entire army, kill all its leaders, destroy all its infrastructure, etc. with no risk of (nuclear) retaliation. There would be many negative consequences too, so it's not likely to happen unless the conventional war is going very badly for Russia, but it's certainly a card they hold.
Putin doesn't know that. Putin doesn't know quite what the repercussions would be. That's why he hasn't used them so far.
The reason the EU countries don’t just waltz into Ukraine probably has more to do with the knowledge they’re not dealing with a rational person, but with a bunch of ego. It’s not hard to predict how Putin will respond if his ego is threatened, but how the rest of Russia will respond to the command to launch nukes.
It’s not unlike the US, except it’s currently still fairly easy to predict that any US commander will just say no when ordered to fire on the EU.
(I expect a first response would be a single city, with warnings to evacuate. A show of what's next.)
The goal would be that Russians assist in taking out an out of control, lunatic who will destroy us all.
Understand, this isn't just about today. Over the next 100 years, every country will obtain the capacity for nukes. Every person on this planet needs to see, and know that using them indiscriminately means you lose.
The consequences are too dire otherwise.
And when a US city is taken out in response? Do you keep going? Over a country that ultimately has very little impact on the US?
Your premise is basically that US military leaders are irrational to the point of insanity.
This isn't a game you win by backing off. There is no safety in that move.
Understand, the claim that the Ukraine is "Russian" is an absurd bluff. No one truly believes that, not Putin certainly. There are a dozen other countries in that region that Putin could make the same bluff about.
By allowing this bluff to continue for this long, matters have only gotten worse. We allowed 2014 to happen. Now, we're here. It won't stop a a peace agreement, or at Ukraine in whole.
The insanity is Russian. And you don't leave a rambling, insane man with a gun outside your house, shooting randomly at houses, because they're not quite at your doorstep yet.
The game theory works roughly like this: Putin wins the most if his threats work. If they don't work, he has the choice between drawing a new red line, launching a limited nuclear attack on Ukraine, or full-on nuclear war. The last option is obviously out (guaranteed destruction is in no proportion to the reward). A limited nuclear attack would likely make him a pariah on the world stage and make it even more difficult to find trading partners. So instead he redraws the line.
If he ever went for the limited nuclear attack option Ukraine might give up, and the west would have to choose between war against Russia (high risk, low reward) or just tightening economic sanctions to the max and punishing anyone who dares to trade with him.
If we assume rational actors I don't see how this would ever escalate to nuclear war. And as long as Ukraine doesn't find a way to decisively push Russian troops out Russia has no incentive to climb the escalation ladder. Even as prolonged war leads to internal instability
Ukraine can cross as many of Putin's "red lines" as they wish. Ukraine did not start this war.
That's purely a psychological trick, if you take this as true then it would be rational for everyone to place madmen in the negotiating seat. A better example, if I had Trump say to China, if you don't open up your markets and drop your tariffs on us, we will nuke you. If I can guarantee as thought exercise that Trump will fire nukes if demands are not met (and that's not difficult to do in reality), do you think Xi Jingping will concede here?
That’s, again, the whole problem. You don’t have to view Putin as “mad” to see him taking a nuclear last stand over Ukraine. He is already invested and taking risk at an extreme level to the point that no one can doubt the importance to him and his willingness to make heavy sacrifices to win. There is no believable narrative where a US leader would be willing to gamble at an existential level over Ukraine.
The logical consequence of disagreeing with this is that every country that doesn't want a significant percentage of their population killed or conquered or enslaved should get their own nukes, because nobody else wants them. And I'm not sure every country acquires nukes is a safer world than maybe we don't let Putin have everything he wants