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NATO forces, intended as defensive, are massing near Russian borders that, with much of Russia’s military bogged down in Ukraine, are unusually vulnerable. Increasingly paranoid Kremlin leaders, faced with economic devastation and domestic unrest, may believe that a Western plot to remove them is already underway.

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The Kremlin has turned to nuclear saber-rattling that may not be entirely empty of threat. Russian war planners, obsessed with fears of NATO invasion, have implied in recent policy documents and war games that they may believe that Russia could turn back such a force through a single nuclear strike...

...A recent Princeton University simulation, projecting out each side’s war plans and other indicators, estimated that it would be likely to trigger a tit-for-tat exchange that, in escalating to strategic weapons like intercontinental missiles, could kill 34 million people within a few hours.

Alexander Vershbow, NATO’s deputy secretary general from 2012 to 2016, said that Western leaders had concluded that Russian plans to use nuclear weapons in a major crisis were sincere...

With Russian forces struggling in a Ukraine conflict that Moscow’s leaders have portrayed as existential, Mr. Vershbow added, “That risk has definitely grown in the last two and a half weeks.”

Since at least 2014, when Russia’s annexation of Crimea led to high tension with the West, Moscow has articulated a policy of potentially using nuclear weapons against any threat to “the existence of the state itself.”

Russian statements have subsequently expanded on this in ways that may make the country’s nuclear tripwires easier to inadvertently cross.

In 2017, Moscow published an ambiguously worded doctrine that said it could, in a major conflict, conduct a “demonstration of readiness and determination to employ nonstrategic nuclear weapons,” which some analysts believe could describe a single nuclear launch.

Evgeny Buzhinsky, a retired member of the Russian military’s general staff, has described the aim of such a strike as “to show intention, as a de-escalating factor.” Some versions call for the blast to hit empty territory, others to strike enemy troops.

[In 2018]...Putin...said that Russia could use nuclear warheads “within seconds” of an attack onto Russian territory — raising fears that a border skirmish or other incident could, if mistaken as something more, set off a nuclear strike.

A 2020 Russian government paper seemed to expand these conditions further, mentioning the use of drones and other equipment as potentially triggering Russia’s nuclear red lines.

These policies are designed to address a problem that Soviet leaders never faced: a belief that, unlike during the Cold War, NATO would quickly and decisively win a conventional war against Russia.

The result is a reluctant but seemingly real embrace of limited nuclear conflict as manageable, even winnable. Russia is thought to have stockpiled at least 1,000 small, “nonstrategic” warheads in preparation, as well as hypersonic missiles that would zip them across Europe before the West could respond.

But Russian military strategists continue to debate how to calibrate such a strike so as to force back NATO without triggering a wider war, underscoring concerns that threading such a needle may be impossible — and that Moscow could try anyway.

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...Moscow also seemingly believes that a sort of NATO-Russia conflict has already begun.

Russian strategic doctrine is designed in part around a fear that the West will foment economic and political unrest within Russia as prelude to an invasion.

With...Putin now facing economic devastation and rising protests, “A lot of the pieces of their nightmare are already coming together,” said Samuel Charap, who studies Russian foreign policy at the RAND Corporation.

In these circumstances, Moscow could misconstrue NATO’s troop buildup, or steps of military support for Ukraine, as preparations for just the sort of attack that Russian nuclear policy is designed to meet.

“Between volunteers from NATO countries, all this NATO weaponry, reinforcement of Poland and Romania,” Mr. Charap said, “they might connect dots that we didn’t intend to be connected and decide they need to pre-empt.”

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...Putin has already said that direct Western intervention in the Ukraine war might trigger Russian nuclear retaliation. Now, each uptick in Western support for Ukrainian forces tests those limits.

“Part of our problem is that I’m not sure we have a clear sense of exactly where the lines are. This is why we’re seeing all the hemming and hawing back and forth with the question of providing aircraft. There’s just uncertainty as to how the Russians would take that.”- Dr. Dmitry Gorenburg, an analyst of Russian military policy.

Jeez, Doc, I would hope when we're not sure "where the lines are" or "how the Russians would take" something, that we would remember the idiom, "Close only counts in horseshoes and nuclear hand grenades."

Dr. Ulrich Kühn, a nuclear strategist at the University of Hamburg in Germany...worried that American domestic politics could play a role as well. Should Russia use chemical weapons or commit some other transgression, American leaders could face overwhelming pressure to retaliate beyond what Moscow anticipates.
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Christopher S. Chivvis, a former U.S. intelligence official for Europe, recently wrote that “scores of war games carried out by the United States and its allies all projected that Mr. Putin would launch a single nuclear strike if he faced limited fighting with NATO or major setbacks in Ukraine that he blamed on the West.

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Both sides know that rapid nuclear strikes could wipe out their military forces in Europe, even their entire nuclear arsenals, leaving them defenseless.

To my understanding that is not correct. The U.S. has a large number of nuclear warheads on its submarines. These nuclear missiles are not as accurate as those launched from land or air (although it is my understanding that if they're launched at "Russia" they would land somewhere in "Russia".) but they are impervious to a first strike.

This means that both sides face an incentive to launch widely before the other can do so first — even if leaders believe that the conflict may have begun in error.