As the war in Ukraine enters its second phase the following are the best game theory derived options for the US to adopt (followed by the reasoning for them) –
1) The US announces that the US Air Force will intervene as a full aerial combatant in Ukraine in the event Russia uses chemical or biological weapons in Ukraine. To enhance the credibility of this deterrent the US should move extra air power to Europe.
2) The US announces that we will “probably retaliate with nuclear weapons” (in exactly those words) if Russia uses tactical nuclear weapons against Ukraine. The US should say nothing else about what, exact, form this potential nuclear response might take.
Reasoning –
The possibility of Russia using tactical nuclear weapons in the war is less of a deterrent to Ukraine now that Moscow has indicated it has (at least partially) genocidal objectives against Ukraine.
Since many Ukrainians will think they will be killed regardless (whether the method is conventional or nuclear) Russia’s nuclear arsenal has now lost much of its deterrent power over Ukrainian decision making.
Because Ukraine will not trust anything Russian negotiators say short of providing clear, verifiable, proof they will adhere to an agreement then Ukraine will not be willing to negotiate a ceasefire unless it wins an even stronger negotiating position through combat in Eastern and Southeastern Ukraine – a military accomplishment which is entirely plausible for Ukraine’s military.
For its part, Russia will also not seriously negotiate unless it wins a stronger negotiating position on the battlefield, or if it has no choice.
What this means for America is that there is no plausible way for us to promote a ceasefire on both parties until one side can use victory to dictate terms.
However, in the meantime America can use options #1 and #2 to increase the chances that the second phase of the war is waged within the boundaries of conventional weapons.
The threat of the US Air Force intervening in Ukraine is an excellent way to deter Russia from using chemical and/or biological weapons in Ukraine because (although these types of weapons are feared by the public) they are actually, in practice, not much more effective at destroying military formations than conventional artillery and missiles are.
In some circumstances chemical and biological weapons are actually less effective than conventional explosives.
Because there is not much real-world battlefield advantage for Russia in using chemical or biological weapons (aside from possible psychological intimidation) while the risk posed by the entry of American airpower against what remains of their invasion force would be very high, this policy will likely deter Russia from actually employing these non-conventional weapons.
The second option has the benefit of lowering the risk of tactical nuclear weapons being used in Ukraine, while simultaneously complicating Russian decision making, without the statement much increasing the risk to the United States.
Without America explicitly committing itself to using nuclear weapons this policy stance would force Putin to adjust his risk calculations in multiple game theory scenarios to the benefit of the United States.
If he will not use tactical nuclear weapons even if he loses in Eastern Ukraine then the statement just gives him an extra reason to do what he already was going to do.
If Putin is willing to use tactical nuclear weapons if he loses in Eastern Ukraine, but he would not if America might retaliate with its own nuclear weapons, then option 2 would deter him from using them.
If he is willing to use tactical nuclear weapons if he loses and if he does not care about the risk of American nuclear retaliation then (counter-intuitively) there would be no change to Putin’s behavior whether America does not threaten nuclear retaliation, or if America does threaten nuclear retaliation.
Another benefit is that just by raising the possibility of nuclear retaliation the decision for Russia to use tactical nukes becomes further complicated because the other members of Russia’s elite do not have exactly the same risk calculus as Putin does.
This elite consists of the military leadership, intelligence agencies, and Russian oligarchs who, as a group, have the collective power to overthrow Putin and who collectively influence the decision to use Russia’s nuclear arsenal.
While it is true that Putin himself would be unlikely to politically survive a defeat in Ukraine, most (if not all) of the remainder of the Russian elite would survive Putin losing the war because they have the power to install someone new as Russian President who can continue acting on their other interests.
What Russia’s elite would not survive is a nuclear exchange with the United States if it turns out we were not bluffing (and under this option there is no way for Russia’s elite to be sure ahead of time if we were bluffing unless Russia crossed the tactical nuke redline).
Of course, if Putin wins the war then the Russian elite will not need to make this calculation because, in that case, they have no incentive to get rid of Putin both because Putin would be able to further consolidate power and a Russian victory would make it too dangerous to launch a coup.
Assuming Russia continues to lose, the more divided the Russian elite is over the risk of a nuclear exchange the harder it would be for Putin to use a tactical nuke, regardless of his own preferences.
The reason for using the word “probably” in option 2 is to raise the possibility of an American nuclear response (and therefore get a good portion of the deterrent benefit that a definite commitment would) without committing America to any course of action while Russia’s leadership could only guess what we might, or might not, do in case Russia uses tactical nukes.
The reason for being as ambiguous as possible about what exact form our response to a Russian tactical nuke would be is because some types of nuclear retaliation are more plausible than others.
If we were to narrow our strike options ahead of time there is a risk Russia would dismiss some of them as a bluff.
But by keeping the exact nature and form of our retaliation unclear Russia would have to consider all types of retaliation, what their effects might be on other decisions, how plausible each type of retaliation is, thus forcing them to factor a larger burden of risk than they would if America were specific.
Pragmatically Distributed 