Although Vladimir Putin's invasion of Ukraine was an act of sheer madness, Putin believed he was acting rationally, mainly because his military had hoodwinked him into believing in its enormous, unbeatable might. Now that the myth has been dispelled, even in Vladimir's mind, he has become what his invasion was: insane. He's cornered and desperate, willing to do anything to protect what he still sees as his seat of imperial power. Because he's also a sociopath.
Hence the U.S. must decide how to act should he deploy a nuclear weapon in Ukraine, which is fast slipping from his grip, and thus diminishing his power at home. U.S. intelligence reports no signs yet of such a deployment — for now, the moving around of nuclear assets — but administration officials and military analysts are of course deep into pondering how the U.S. should act.
Parenthetically, the world mustn't be fooled into thinking that "mere" tactical nukes are somehow less destructive than their bigger brothers. Even Russia’s "smallest warheads have many times the explosive power of the bombs dropped on Hiroshima and Nagasaki," notes NBC News. Paraphrasing nuclear expert Jeffrey Lewis, "there is no technological distinction between 'tactical' nuclear weapons and 'strategic' ones — the difference is in the targets and the goals. Tactical nukes are used to gain advantage on the battlefield, while strategic weapons are aimed at military infrastructure and even whole cities."
Neither would Putin's "test" of a tactical nuke as a seemingly lesser measure be of considerable less danger. Such a test as a show of determination, for example over the Black Sea, would send radioactive material wafting into NATO countries, and probably Russia itself.
The U.S.'s options, according to NBC: "Stay the course, up the sanctions and keep arming the Ukrainians, while building an international coalition against Russia that completely isolates the country; launch a conventional military attack on Russian forces in Ukraine or Russia; or respond with a nuclear attack."
Without putting a percentage on this held opinion, NBC reports that "some" intel and military analysts said that U.S. military retaliation would be unlikely "after a single Russian use of a so-called tactical nuclear weapon in Ukraine." Others — again, we know not the number — argued that President Biden would need to unveil at least some sort of conventional-force reaction — "perhaps attacking Russian troops in Ukraine or the Russian military unit that launched the nuclear weapon."
That, however, could have what the analysts described as "serious repercussions, since Russian military leaders might be killed." But what of the repercussions from Russia launching the first nuclear attack since 1945? you might ask. My answer would be, I'm as mystified as you are by their calculation.
A more upbeat opinion comes from former Pentagon official Evelyn Farkas, who said that should Putin issue such a mad military response to losing more Russian-occupied territory, "[He] will be signing the order on changing the regime." On the other hand, two sitting U.S. officials ventured that "Unless they use them on NATO we’re probably not going to respond militarily."
However the U.S. might respond, the Biden administration is playing it close to the chest, leaving open all the above options.
I question the wisdom of this strategy. Putin believes — because he has good empirical reason to believe — that the U.S. will never respond aggressively, even if he detonates a nuclear weapon over Ukraine, slaughtering countless civilians. But if Putin were notified otherwise beforehand, he might rethink his belief.
Somewhere in his mind's swirling madness, there remains an element of sanity. Tell him now that the U.S. will indeed retaliate forcefully (but conventionally) against Russian assets in Ukraine, and he might back off. Because his closest, rational advisers, knowing of the Putin-induced catastrophe to ensue, would have the knives out. And the impersonating Czar — self-preserving sociopath that he is — would know it.