Nuclear Weapons Always Stopped Invasions. Then Ukrainian Troops Poured Into Russia.
In testing Putin’s red lines, Kyiv is sparking a rediscovery of Cold War-era ideas about nuclear escalation
Ukraine’s incursion into Kursk isn’t just a brash bid to upend Russia’s invasion. It also marks the first time that a declared nuclear power has faced invasion and occupation by another country.
For decades, nuclear-escalation theory has presumed that countries with atomic weapons were largely immune from attack because an aggressor risked triggering armageddon. Relatively small states including Israel, Iran, North Korea and Libya have pursued nuclear arms in part to deter attacks by larger, better-armed adversaries.
Nuclear powers have scuffled: India has had border skirmishes with China and Pakistan. Palestinian Hamas militants in October stormed into Israel, which is widely believed to have nuclear weapons. But generally the threat of annihilation has protected nuclear-armed countries from large-scale attack and kept peace between them.
Ukraine isn’t a nuclear power and is outgunned by Russia, yet Kyiv has managed for more than three weeks to control territory now totaling almost 500 square miles. It is a stunning twist. Strategists over the years have frequently envisaged countries from the North Atlantic Treaty Organization grabbing Russian turf in a fight, not a beleaguered underdog doing it.
Now Western leaders, military thinkers and nuclear theorists are puzzling over what current events mean for prospects of Russian escalation—and for future war gaming. Theoretical risk faces a real-world test, forcing a re-examination of the role nuclear weapons can play in deterrence.
Russia’s published nuclear doctrine says Moscow would only resort to atomic weapons if the country’s sovereignty or territorial integrity were threatened. Although Ukraine occupies a chunk of Russian soil, neither side appears to consider the Kursk region strategically vital, so Ukraine’s attack—however embarrassing to the Kremlin—shows no sign of crossing a Russian red line.
But ambiguity and uncertainty are integral to nuclear gamesmanship.
“No one really knows the Russian red line—they’ve never given any precision,” said Nikolai Sokov, a former Soviet and Russian arms-control negotiator. “We may find out later that we crossed the red line two months ago,” said Sokov, who now briefs Western military leaders on Russian strategic thinking.
One wild card Sokov notes is that the Kremlin and President Vladimir Putin seem to consider threats to his regime as sovereign threats to Russia. In a sign of how events are affecting deterrence calculations, Russian Deputy Foreign Minister Sergei Ryabkov on Sunday said that the Kremlin would adjust its nuclear doctrine based on analysis of the war and the West’s role, according to state news agency TASS, reiterating a pledge Putin has made several times.
Fear of crossing Russian red lines has shaped President Biden’s approach to the war. Not wanting it to become a direct fight with NATO, he has hesitated to give Ukraine weapons it has begged for, including tanks, advanced missiles and jet fighters. Kyiv eventually got most of them, prompting Ukrainians and their Western supporters to argue that Putin’s red lines were flexible.
Ukraine aims to show with its Kursk incursion that another taboo can be broken without dire consequences. Part of the aim is to convince the White House that Ukraine should be allowed to use more lethal and precise U.S. weapons to attack Russia.
Many Western officials, particularly in Washington and Berlin, remain more cautious because Putin is so unpredictable.
Uncertainty over where Russia’s red lines lie is “the fundamental challenge of strategic ambiguity,” said Janice Gross Stein, a professor of conflict management at the University of Toronto. Testing limits and signaling through threats or pledges of restraint form what she calls a “contest between a strategy to manipulate uncertainty and a strategy to reduce uncertainty.”
Gaming out events triggered by Ukraine’s push into Kursk harks back to the Cold War, when escalation theory was a widely studied discipline. When the Soviets developed an atomic bomb in 1949, four years after the U.S. had, Western strategists tried to envisage how the terrifying weapons might figure into combat.
Their massive destructive power—and the far greater danger from thermonuclear weapons, starting in 1952—prompted creation of a new field, nuclear deterrence thinking, and analytical tools to support it.
Game theory, pioneered in the 1920s by mathematician John von Neumann, flourished as an approach to assessing nuclear brinkmanship. To tackle weighty unknowns, the Rand Corporation, created as a Pentagon think tank, used theoretical constructs such as the so-called prisoner’s dilemma—a situation where two parties that are unable to communicate must separately decide whether to cooperate for mutual benefit—to play out how the U.S. and Soviet Union might act and react in conflict scenarios.
Theorist and economist Thomas Schelling in 1960 codified the analysis in a deeply influential collection of scholarly articles, “The Strategy of Conflict,” which presented an intellectual path for assessing deterrence and escalation.
Nuclear theorizing drew brilliant thinkers. Schelling won a Nobel Prize in economics. Escalation theory was advanced by the work of Nobel-winning economist John Nash, subject of the film “A Beautiful Mind.”
Yet nuclear strategizing and determining an adversary’s red lines remain a high-stakes game of chicken.
“It is like we’re walking in the dark toward a cliff,” said Christopher Chivvis, who has assessed nuclear-risk issues at Rand and as a U.S. intelligence officer. “It is out there somewhere. We just don’t know where it is.”
Putin has more or less directly threatened to use nuclear weapons at several points since launching his full-scale invasion of Ukraine in 2022. But already a decade ago, Russia’s seizure of Ukraine’s Crimean Peninsula marked a new level of post-Cold War aggression that prompted escalation theorists to dust off their textbooks.
About six years ago, political science professor James Davis, chair of international relations at the University of St. Gallen in Switzerland, realized that nuclear decision-making had faded as an academic field but was returning as a concern. He began teaching courses on it, including one titled “Fundamentals of Arms Control.”
Now he’s reassessing some long-held assumptions.
“We always thought nuclear weapons weren’t good for anything but deterrence,” he said. “We really didn’t think a nonnuclear power would invade a nuclear power.”
Communicating uncertainties of escalation theory outside the classroom, to the general public, can be even more puzzling. Specialists note that 75 years of successful deterrence doesn’t mean the approach can always prevent nuclear war.
“It is a difficult messaging task for people who think about this full-time,” said Chivvis, who is now a senior fellow at the Carnegie Endowment for International Peace. “You don’t want to be warning about this all the time, but you don’t want to say it’s not going to happen.”
Write to Daniel Michaels at Dan.Michaels@wsj.com