Recent conflicts have challenged a traditional view of nuclear deterrence, which proposes that no entity would launch an attack, nuclear or otherwise, on a nuclear state for fear of that state’s retaliatory power. Since Russia’s full-scale invasion of Ukraine, neither Russia’s nuclear arsenal, the largest in the world, nor President Vladimir Putin’s repeated allusions to potential nuclear escalation has deterred Ukraine from striking Russian military bases and cities, including Moscow. Last summer, Ukrainian troops even seized some 500 square miles of territory in Russia’s Kursk region, a portion of which they continue to hold. Similarly, Israel’s nuclear capability failed to dissuade Iran from launching missile attacks on that country in April and October 2024. Earlier last year, Iran also struck members of a Sunni militant group operating in Pakistan, another nuclear state. In each of these cases, the ultimate weapon, thought to be the ultimate deterrent, appeared to carry little threat.
The nuclear shadow has precluded large-scale war between nuclear states. A fear of mutual destruction helped keep the Cold War between the Soviet Union and the United States from escalating to direct conflict, for instance. And decades of deadly clashes between India and Pakistan shrank to a restrained conflict followed by smaller skirmishes after both sides tested nuclear weapons in 1998.
But nuclear arms have long had a spotty track record when it comes to deterring conflict between a nuclear state and a nonnuclear one. There are extensive drawbacks to using nuclear weapons, including their very destructiveness—which could undermine larger objectives or complicate battlefield operations—and the international backlash that would follow. Since World War II, many nonnuclear states have recognized that their nuclear adversaries face such constraints and have thus felt emboldened to attack, correctly surmising that inflicting significant casualties on a nuclear power and even taking some of its territory would not trigger nuclear retaliation. Short of a large-scale threat to its homeland or the collapse of its military, a nuclear-armed state will likely remain reticent to deploy nuclear weapons against a nonnuclear rival. In these scenarios, the most powerful weapons ever built confer limited practical advantages.
CAUSE AND EFFECT
States considering nuclear strikes confront an array of obstacles, regardless of whether their opponents have nuclear weapons. The destructiveness of even modest-yield nuclear weapons, particularly those detonated on or near the ground, can compromise grander plans; it makes little sense to destroy an area if the goal is to acquire it, take its resources, or liberate its population. Furthermore, the fallout of nuclear strikes against a neighbor could quite literally blow back to harm the attacking state. In some cases, strikes could complicate conventional military operations by contaminating the battlefield.
Detonating very-low-yield nuclear weapons at altitude, which would inflict less collateral damage, could be a way around such hurdles. But limiting destruction can mean limiting military effectiveness, as well. For instance, in 1990 during the leadup to the Gulf War, Dick Cheney, then the U.S. secretary of defense, asked Colin Powell, the chairman of the Joint Chiefs of Staff, to look into how many tactical nuclear weapons it would take to destroy an Iraqi Republican Guard division. Powell commissioned a report and found that the answer was 17. Powell later wrote that “if I had had any doubts before about the practicality of nukes on the field of battle, this report clinched them.” Even in an ideal environment for low-yield weapons—in the desert, away from population centers—it would take far too many of them to accomplish a straightforward goal.
If a state deploys a nuclear weapon, it will face international consequences. Any number of countries, whether they want to uphold a long-standing norm against nuclear use or are simply friendly to the target country, could politically or financially isolate the attacker, say, or come to the target’s aid. Crucially, launching a nuclear strike could also spur countries that don’t currently have these weapons to nuclearize for their own self-defense.
A nuclear arsenal is an insufficient deterrent against nonnuclear opponents.
Despite the drawbacks, the prospect of a major battlefield defeat has occasionally prompted debate about possible nuclear use. In early 1968, for instance, North Vietnamese troops besieged U.S. marines at Khe Sanh in an important battle of the Vietnam War. Senior officials in the Lyndon Johnson administration discussed using tactical nuclear weapons against the North Vietnamese if the situation deteriorated, but Johnson shuttered the idea when news of it leaked. When Ukrainian troops broke through Russian lines in the fall of 2022, Russian military commanders also weighed the use of nuclear strikes, according to U.S. intelligence, but there is no evidence that Russia was preparing its weapons for such a possibility. In both cases, the nuclear power did not face a real threat to its survival and avoided a military collapse, two informal but critical thresholds for nuclear use.
When two nuclear powers that could devastate one another clash, the danger to each is existential. But a nonnuclear state is generally less capable of imperiling the survival of a nuclear foe. This relative weakness gives nonnuclear states more freedom of action, and historically they have pushed further against nuclear opponents than nuclear states have against one another. Usually, a nonnuclear state employs strategies to further reduce the risk of nuclear retaliation. When China intervened in the Korean War in 1950 and inflicted major losses on U.S. forces, for instance, Chinese leaders sought Soviet air support to deter a potential U.S. nuclear attack on Chinese territory. In their coordinated attack on Israel in October 1973, Egypt and Syria planned offensives with limited scopes, and Egyptian leaders communicated the limits of the Egyptian attack so that Israel would not overestimate the scale of the threat.
Despite its strikes on Russia and its incursion into the Kursk region, Ukraine, too, ultimately lacks the capability to threaten Russia’s survival, a reality consistent with past examples of nonnuclear states fighting nuclear ones. Consequently, Putin has said he does not see a need to use nuclear weapons in Ukraine, even though he has engaged in some nuclear saber rattling. Instead, Russia has inflicted large-scale destruction on Ukraine with conventional military forces.
SOMEONE YOUR OWN SIZE
Russia has behaved more cautiously toward the United States and other NATO countries. Both Russia and Western powers have pursued policies that harm the other, such as cyberattacks and sanctions. Yet Russia has refrained from military strikes against NATO countries, despite their supply of weapons to Ukraine that have inflicted large Russian losses. The United States, too, has ruled out direct intervention, even when Joe Biden, during his presidency, expanded the category of weapons that the United States supplied to Ukraine and eased restrictions on their use. President Donald Trump has shown no interest in direct conflict, either.
Nuclear states compete intensely with one another, but this competition almost never escalates to large-scale conflict. The security-studies scholar Glenn Snyder speculated in 1965 that deterrence of the most destructive forms of nuclear conflict would create instability at lower levels of confrontation, including conventional war and limited nuclear use. But the deterrent effect of nuclear weapons among nuclear states has turned out to be even more extensive than Snyder suspected. Mutual vulnerability induces greater caution, making war of any kind less likely.
Among the many reasons is that when both sides possess nuclear arsenals, even trading limited strikes has the potential to escalate all the way up the ladder to use of the world’s most powerful bombs. Another is the possibility that early in a conflict, one or both nuclear powers may anticipate escalation and try to limit damage to themselves by taking out the opponent’s nuclear forces. A state may thus determine that the costs of inaction outweigh the costs of nuclear use: why worry about complicating military operations or upending norms when the survival of multiple cities may be at stake? Even if both sides are seeking to avoid using nuclear weapons, events could escalate inadvertently, and the dangers of accidents, miscalculations, and unexpected developments—all common in war—leading to nuclear strikes are larger when both sides have nuclear weapons. Lest they find themselves in these enormously risky scenarios, nuclear states have tended to avoid fighting one another in the first place.
Among nuclear states, mutual vulnerability induces greater caution.
Nuclear powers’ recognition of the dangers of spiraling escalation has helped ease tensions in past high-stakes moments. During the 1962 Cuban missile crisis, for example, the Soviet leader Nikita Khrushchev warned U.S. President John F. Kennedy that “if indeed war should break out, then it would not be in our power to stop it, for such is the logic of war.” Six Kennedy administration officials later recalled that the “gravest risk” at the time “was not that either head of government desired to initiate a major escalation but that events would produce actions, reactions or miscalculations carrying the conflict beyond the control of one or the other or both.” Those fears compelled both leaders to defuse the crisis.
India and Pakistan’s direct conflict during the Kargil War in 1999, by which time both were nuclear powers, is a rare example of nuclear states’ waging war on each other. Yet this war is less of an exception than it may appear. The specter of uncontrollable escalation is a major reason that nuclear states avoid fighting each other, but according to the international relations scholars Mark Bell and Julia Macdonald, the danger of this in the Kargil War was low: the fighting was contained to the mountainous Kashmir region, India normally kept its nuclear warheads separated from the weapons that could deliver them, and both countries communicated clearly with each other. Still, a fear of nuclear use played a significant role in containing the fighting. Since then, skirmishes have continued but never developed into war.
Political scientists often talk about the theory of democratic peace, which posits that democratic countries don’t fight wars with other democratic countries. Democracies, however, are not inherently peaceful, and they fight plenty with nondemocratic states. This basic premise has a nuclear analogue: nonnuclear states will fight nuclear opponents, but between nuclear states there is a fragile, often contested, yet enduring peace. Analysts and policymakers should account for these two measures of nuclear deterrence when they assess the nuclear risks of today’s conflicts. A nuclear Iran, for example, may be emboldened in the Middle East, but the history of the nuclear age suggests that the threat of nuclear escalation may also compel Tehran to behave more cautiously toward Israel than it has to date. Russia has refrained from launching a nuclear response to Ukrainian strikes, but its reaction might well be different if U.S. forces were the ones firing at Russian targets.
There is much that remains uncertain about the nuclear age. The paucity of evidence—there have fortunately been only two nuclear strikes and no nuclear wars—should generate some caution in drawing sweeping conclusions about nuclear scenarios, which are all speculative. But it is still possible to take away tentative lessons. The nature of nuclear weapons creates significant downsides to their use; for the past three-quarters of a century, these risks have deterred major conflict between nuclear states. But time after time, a nuclear arsenal has proved an insufficient deterrent against a determined nonnuclear opponent with limited goals. Nuclear states need conventional tools to counter such threats, rather than rely on a fear of the world’s most dangerous weapons.
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