Why the US won’t send troops to Ukraine

US soldiers training in Germany on January 27, 2022. | Armin Weigel/Picture Alliance/Getty Images

Nuclear weapons are containing the Ukraine war. They also helped cause it.

In 1990, Iraq invaded Kuwait in a naked war of territorial aggression. The next year, the US and an allied coalition intervened under the auspices of the United Nations Security Council, repulsing the Iraqi invasion. Today, as Russia is engaged in a similar aggressive war against Ukraine, there is no similar American effort in the offing — even as Ukrainian leaders have pleaded for Western assistance.

There are many dissimilarities between the situations in 1991 and 2022, but the biggest one is this: Saddam Hussein, rather famously, did not have nuclear weapons. Vladimir Putin has approximately 6,000 of them. And that makes all the difference.

Both before the invasion and afterward, the Biden administration has consistently ruled out the deployment of US troops. “Let me say it again: Our forces are not — and will not — be engaged in the conflict with Russia in Ukraine,” the president said in a Thursday address. Despite the warnings of American involvement from commentators on the Trumpist right and “anti-imperialist” left, there are no signs of this policy changing. Nuclear weapons are the chief reason why.

The logic of mutually assured destruction that defined the Cold War still works, to some degree: Russia’s arsenal makes any direct intervention in Ukraine riskier than any rational American leader could tolerate. In a sense, then, Russia’s nuclear weapons make it less likely that the conflict will kick off World War III.

But in another sense, Russia’s nuclear arsenal also helped create the conditions where Putin’s invasion could happen in the first place.

Political scientists call this the “stability/instability paradox,” the notion that nuclear deterrence has had the paradoxical effect of making certain kinds of conventional warfare more likely. Russia can be relatively confident that the United States and its allies won’t come to Ukraine’s defense directly, because such a clash carries the threat of nuclear war. This could make Putin more confident that his invasion could succeed.

Putin himself has suggested as much. In his speech declaring war on Wednesday night, he warned that “anyone who would consider interfering from the outside” will “face consequences greater than any you have faced in history” — a thinly veiled threat to nuke the United States or its NATO allies if they dare intervene.

“This is about the clearest evidence I have ever seen for the stability-instability paradox,” Caitlin Talmadge, a professor at Georgetown University who studies nuclear weapons, writes of Putin’s speech. “Putin’s behavior suggests that revisionist actors [can] use their strategic nuclear forces as a shield behind which they can pursue conventional aggression, knowing their nuclear threats may deter outside intervention.”

The nuclear balance between the United States and Russia, one of the Cold War’s defining features, is coming back to the forefront of international politics. We can only hope that things don’t get scarier from here.

How nuclear weapons make US involvement in Ukraine unthinkable

Nuclear weapons are the only weapons humanity has yet devised that, deployed at scale, could swiftly wipe out our entire species. The risks of conflict between two nuclear-armed powers are so great that virtually any rational leader should, in theory, seek to avoid one.

This is especially true of the United States and Russia, who together control an estimated 90 percent of the world’s nuclear warheads. The issue is not merely the size of their arsenals but also their structure — both countries have robust “second strike” capabilities, meaning each can sustain a devastating nuclear first strike from the other side and still retaliate. The US and Russia maintain second strike capabilities in part through the so-called “nuclear triad”: bombers armed with nuclear bombs, submarines equipped with nuclear missiles, and land-based missile launchers.

The result is that neither the US nor Russia can hope to “win” a nuclear war. Even if one nation struck first, decimating major military bases and population centers, the other would still be able to launch a devastating nuclear counterattack on their enemy’s homeland from (for example) submarines out to sea. The only way to win is not to play.

This appears to be the reason why the Biden administration has been so adamant on avoiding any kind of involvement in Ukraine; the risks of any direct intervention are far too high.

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An image of an American nuclear test in 1954.

Conventional warfare between nuclear powers does not necessarily escalate to nuclear conflict: see the 1999 Kargill conflict between India and Pakistan, the 2018 battle between US special forces and Russian mercenaries in Syria, or the recent border clashes between India and China. But the risk of such a conflict escalating to nuclear use is always there, especially if one side believes that vital national interests or its very survival is at stake.

For Putin, the Ukraine war seems to fit the bill. A significant US or NATO intervention in the conflict would, by sheer fact of geography, pose a threat to the territorial integrity of the Russian homeland. Were it to turn the tide of the war in Ukraine’s favor, Russia could very conceivably use its nuclear arsenal against its NATO enemies.

“Their nuclear strategy envisions possible first use if they are losing a conventional conflict or facing an existential threat,” Nick Miller, an expert on nuclear weapons at Dartmouth University, explains.

We have no guarantee that deploying US troops to Ukraine would, in fact, lead to nuclear warfare. But the risks would be high, very likely exceeding the most dangerous moments of the Cold War, like the Cuban missile crisis. There are scenarios where you could imagine an American leader launching a conflict with a nuclear power — if it was necessary to protect the US homeland, for example — but defending Ukraine, which isn’t even a formal US ally, simply isn’t one of them.

How nuclear weapons helped make the Ukraine war possible — and could make it much worse

Some leading scholars look at the logic of deterrence and conclude that nuclear weapons are actually a good thing for the world. This “nuclear revolution” theory, most commonly associated with the late political scientist Kenneth Waltz, holds that the spread of nuclear weapons will spread peace by expanding deterrence. The more countries can make aggression unthinkably risky, the less likely war will become.

The evidence for this theory is spotty. While nuclear deterrence does seem to have played a role in preventing the Cold War from turning hot, examining other cases — including smaller nuclear armed states like India, Pakistan, Israel, and North Korea — leads to a much more complicated picture.

The stability-instability paradox is one of these complications. In its most classic form, the paradox argues that two countries with nuclear weapons can be more likely to engage in small-scale conflict. Because each side knows that the other doesn’t want to risk a wider war given nuclear risks, they can feel more confident engaging in smaller provocations and assaults. What looks like nuclear stability actually breeds conventional instability.

Ukraine is not a nuclear state, but the NATO alliance has three of them (the US, Britain, and France). Because NATO states don’t want a wider war with Russia, one that carries a risk of a nuclear exchange, they’re less likely to intervene in a conflict they might otherwise join. Putin knows this; his public threat to use nukes against any intervening country suggests he’s counting on it.

So what we’re seeing is a kind of twist on the classic paradox: Putin is relying on nuclear fear to allow him to get away with invading a country (Ukraine) that a nuclear-armed third party (NATO) might otherwise want to defend.

This dynamic is familiar from the Cold War; it’s in part why the Soviets could send troops to Hungary in 1956 and Czechoslovakia in 1968 to suppress popular anti-communist uprisings without real fear of Western intervention.

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A Soviet Army unit in Czechoslovakia in 1968.

To be clear, the stability-instability paradox is not an ironclad law of international relations; scholars disagree about exactly how frequently it actually causes conflict. But neither is nuclear deterrence: there are several near-miss examples where a nuclear exchange was just barely avoided.

In 1983, for example, Soviet Lt. Col. Stanislav Petrov was alerted by an early warning system that a US nuclear strike was likely incoming. Had Petrov informed his superiors of that message, it’s very likely they would have launched missiles in response. Yet Petrov and his staff correctly concluded this was a false alarm and chose to say nothing — potentially saving hundreds of millions, if not billions, of lives.

Nuclear deterrence depends on both sides having good information and making rational decisions. But in a conflict like the one we’re seeing in Ukraine, taking place near the borders of NATO members, the risks of accidents, misperceptions, and miscalculations inches incrementally higher. For example, says Miller, “you can imagine a Russian jet straying into NATO airspace accidentally” and sparking a wider conflict.

Without a NATO presence inside Ukraine, the risks of such a disaster remain extremely low: Miller cautions that “both sides have a strong incentive to avoid direct conflict and avoid minor incidents escalating.”

But the fact that we’re even talking about it illustrates how nuclear weapons, by their very nature, make the world a riskier place. While they likely are playing a major role in keeping the US out of the Ukraine conflict directly, they helped create the conditions where Russia could launch the war in the first place — and, in the very worst case, could escalate to complete disaster.