What would happen if Russia decided to deploy tactical nukes in Ukraine?

The "Budapest Memorandum"[1] article 4 would be invoked:

4. The Russian Federation, the United Kingdom of Great Britain and Northern Ireland and the United States of America reaffirm their commitment to seek immediate United Nations Security Council action to provide assistance to Ukraine, as a non-nuclear-weapon State party to the Treaty on the Non-Proliferation of Nuclear Weapons, if Ukraine should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used.

That doesn't really answer the question, however, since "seek immediate United Nations Security Council action to provide assistance to Ukraine" is not a very strong statement.

NATO would not be directly involved (IMHO) because NATO is a mutual defense treaty and (hypothetically) no NATO country would be attacked. However, spillover from an attack into / onto a NATO country would (theoretically) draw all of NATO into the war.

[1] https://en.wikipedia.org/wiki/Nuclear_weapons_and_Ukraine#Bu...

NATO (US) - Russia deterrence model rests on nuclear deterrence, and most importantly on the fact that Russia can guarantee MAD in retaliation against US because they have a huge stock pile of them. This deterrence model is not itself changed if Russia detonates a nuke in a non-NATO country. If it were, we could just skip past that and just send nukes to their military installations right now.

That said it's only a deterrence model if you have stone cold people like the people who set up the MAD system. It might break when it is lead by hysterical people and then MAD becomes "acceptable cost".

And I make it a 0 probability event that Russia uses a nuke in Ukraine. They're winning, they're using limited warfare (ie. no mobilization in RF), NATO probably follows the nuclear deterrence calculus, and they see the Ukrainians as part of the Russian people (https://en.wikipedia.org/wiki/All-Russian_nation) based on Putin's speeches.

A NATO country - USA or UK - will send a tactical nuclear rocket to a Russian military center, on Russian territory or elsewhere. Ukraine will get advanced weapons, like aircrafts and seaships, during a week since then. NATO might actually send troops to Ukraine. Black Sea - and likely Azov Sea - will see much more of American fleet. West will announce closing air over the Ukraine. All tradings with Russia will stop. Economical sanctions become harsher abroad - things like Rusal problems when Deripaska went under sanctions last time won't stop new efforts. Sweden and Finland will get in NATO and the "new" NATO will put some forces - more forces - in those countries.

Well, nobody of course can be certain about what would happen. It seems unlikely West will let it slip unanswered though.

A single tactical nuke wouldn't be decisive (do vastly more damage than conventional weapons.) It would be decisive to use 50 of them.

A nuclear bomb with a 1kt or so yield kills people within a radius of about 1km with neutrons; at that radius there is some damage to buildings, but you don't die because the building collapsed. The neutrons do a good job of penetrating walls and armor so it is effective against tanks.

A B-52 can carry about 35 tons of bombs, so it would take about 30 sorties to deliver that much tonnage -- precision bombs like

https://en.wikipedia.org/wiki/GBU-39_Small_Diameter_Bomb

get closer to their targets than the nuke does, so the number of sorties to do an equivalent amount of damage would be a lot less than 30.

You've seen the massive damage to residential districts done by cluster bomb, thermobaric and other rocket artillery. Thermobarics have at least 3x the impact of conventional explosives, so I'd imagine that it's routine for large groups of rocket artillery to do "kilotons" worth of damage.

>> What would happen...

The probability of nukes hitting Moscow, New York, Paris, and London in the next 48 hours would skyrocket.

One of the lowest-levels of escalation that Nato could take in response would be to deploy conventional assets into Ukraine, particularly fighter jets and anti-air systems. If Russia wanted to maintain a military presence in Ukraine, it would likely have to use further tactical nukes against Nato assets. Nato would have to respond in kind in order to maintain a credible deterrent. Very quickly you have a tactical nuclear exchange which can escalate (in minutes) to a strategic exchange. 15 minutes later all of the cities we know and love are gone. There's a non-negligible chance that the situation escalates to a global exchange including China, North Korea, Japan, and Australia. We can only hope that some parts of the third world are spared from which humanity can once again flourish. Even then, the nuclear winter likely kills an additional billion people due to mass crop failure and famine.

Much of strategic deterrence (the formal term for nuclear deterrence) is analyzed and approached through game theory. So much so that the US DoD's entire strategy rests on 1 simple assumption:

"Every operational plan in the DoD, and every other capability we have, rests on the assumption that strategic deterrence, and in particular nuclear deterrence, will hold. If strategic or nuclear deterrence fails, integrated deterrence and no other plan or capability in the DoD will work as designed. The Nation’s nuclear forces underpin integrated deterrence and enable the U.S., our Allies and partners to prevent and, if necessary, confront aggression around the globe using all instruments of national power"

Source: https://www.stratcom.mil/Portals/8/Documents/2022%20USSTRATC...

The threat of Russia's deployment of non-treaty tactical weapons on a non-NATO state has been carefully analyzed for a long time: https://sgp.fas.org/crs/nuke/RL32572.pdf

The most credible reason for Russia to commit to such a deployment of weapons is if Russia believed that the deployment would ultimately lead to a de-escalation of a conflict ("escalate to de-escalate").

At this point, the US/NATO has made clear publicly that any escalation to WMDs/strategic weapons will lead to a response in kind. A response in kind would not lead to de-escalation, and thus Russia's threat isn't really credible other than for information warfare purposes.

But of course, much of game theory rests on the assumption of common knowledge of rationality. This is why the diplomatic/intelligence establishment is so concerned with the state of Putin's rationality.

Why would Russia do that? The risk of radioactive fallout drifting over Russia would be enormous.

That's in the same league as shooting yourself in the foot. Sure, you can do it, but it's not worth the damage to yourself.

Putin already did.

Threatening nuclear war if other countries don't let you rape, murder, and pillage the people of a nearby country is using nukes.

The real question is, what is the rest of the world going to do about it?

If Russia deployed tactical nukes in Ukraine why would they not also use nukes against supplies staging points in NATO countries, or even against the nuclear forces of NATO countries? Once you cross the line and go nuclear does it much matter if you're "only" using tactical nukes? If they did that I suspect a strategic first-strike would happen at the same time.