In the short run, Groves was wrong, while Oppenheimer’s guess was correct. The Hiroshima uranium bomb was in fact more powerful than 1,000 Tallboys, with the Nagasaki plutonium bomb exceeding even that. But only five years later, Groves’s prediction came to pass. First the United States, and then the Soviet Union, and then each successive nuclear power came to the realisation that their nuclear weapons were too powerful to be used in combat. This has remained true in the decades since — all the way up to the invasion of Ukraine. For, despite Putin’s atomic sabre-rattling, he too is subject to the logic of Groves’s prediction. Decades after his conversation with Oppenheimer, a brief historical summary of nuclear war has much to teach us about the situation in Ukraine — and how victory might only be attainable there through much more conventional means.

The first test of the nuclear age came with the Korean War. In December 1950, hundreds of thousands of Chinese soldiers crossed the River Yalu to support their North Korean allies against the US. With America in immediate danger of losing tens of thousands of men, General Douglas MacArthur decided that he had to use nuclear weapons to stop the Chinese. By far the most respected US military leader of the time — he had led American forces in the Pacific from humiliating defeat to total victory, and then acted as Japan’s de facto Emperor in reforming the country — MacArthur expected Truman to assent to his superior military judgment. Instead, the answer was a flat no. MacArthur insisted, and he was dismissed.

Truman recognised that the nature of warfare had fundamentally changed since Hiroshima and Nagasaki. When he authorised those strikes, neither he nor anyone else knew that the explosions would also cause radiation fallout, which would sicken and even kill thousands of people miles from the site of the detonation. Moreover, in 1945, Truman was facing the prospect of losing many more American troops in the conquest of Japan than in the entirety of the Second World War up to that point. The Japanese really did fight to the last man, and still had 2 million troops to expend. Truman would have been thrown out of the White House if he had allowed the deaths of hundreds of thousands of Americans by refusing to use the bomb.

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But five years later, the situation was very different. Facing catastrophe in Korea, Truman had the alternative of evacuating US troops to Japan if all else failed — and therefore never even considered using atomic weapons. Under the next president, his fission bombs evolved into thermonuclear fusion bombs at least 100,000 times more powerful than Tallboy. But that only made Truman’s “No” of 1950 even more definitive. Nuclear abstinence became the only possible choice for Americans and Russians alike, as the Cuban Missile Crisis precariously but definitively showed.

However, it would take much longer for this logic to develop into a definitive doctrine. Following the establishment of Nato 75 years ago today, and especially in the Sixties and Seventies, exhaustive efforts were made to extract some additional advantage from nuclear weapons and somehow gain the upper hand for the new Western alliance. So-called “tactical” nuclear weapons were made not more, but much less powerful, supposedly to enable their use on the battlefield. Their advocates claimed that they could provide firepower very cheaply, with small nuclear warheads replicating the effect of hundreds of howitzers. Both the US and Soviet armed forces duly acquired thousands of nuclear weapons: not only “small” bombs for fighter-bombers, but also bombardment rockets (some small enough to be carried in a jeep), anti-aircraft missiles, torpedoes, and even portable demolition charges.

But this illusion could not be sustained. Military planners came to understand that if US commanders tried to defend Nato territory by attacking invading Soviet forces with small “tactical” nuclear weapons, the Russians would use their own arsenal to destroy the defending Western forces. The same would apply for any attempt to replace conventional military force with nuclear weaponry. And so it was understood that, while nuclear weapons are a useful deterrent, they can only be used to strike back against a prior nuclear attack — and never to achieve any kind of victory. Thus in the Seventies, when the United States and the Soviet Union were engaged in the elaborate and highly publicised “Strategic Arms Limitation” negotiations, officials on both sides quickly agreed to quietly stop developing, manufacturing and fielding new “tactical” nuclear weapons, before equally quietly disassembling tens of thousands of these weapons.

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But in the end, it was the newest nuclear powers, India and Pakistan, that conclusively demonstrated the redundancy of their own nuclear weapons for anything beyond reciprocal deterrence. In the Kargil War of 1999, which involved multiple full-scale battles and thousands of casualties, neither side attempted even a sotto voce threat of a nuclear strike. And this is still true today. When Putin‘s loudest attack dog, Dmitry Medvedev, started barking about the use of “tactical” nuclear weapons after the failure of the initial Russian invasion in 2022, it was only the least competent journalists and those obedient to Moscow who echoed his warnings. Eventually, after several months of this foolishness, Putin came out and said it: Russia would only use nuclear weapons only “when the very existence of the state is put under threat” — meaning by a corresponding nuclear threat.

The situation in Ukraine has turned again, but the same logic holds. Instead of frustrated Russians bogged down in their trenches, now it is the Ukrainian position that looks precarious. Kyiv presents this as a question of materiel, and continually asks for more and better weapons from the West. Yet while more guns and missiles could be sent, it is clear that what is forcing Kyiv to retreat step by step is not a lack of firepower, but a lack of soldiers.

Until this week, conscription in Ukraine only started at the age of 27, as opposed to the global norm of 18. Zelensky has now reduced this to 25; but with many Ukrainians exempted from service, his total armed forces amount to less than 800,000 active personnel. Ukraine is hampered by the age distribution of its population, with children and older people over-represented when compared to young men in the 19-35 age bracket. But its troop total is still too low for a population that exceeds 30 million by most estimates, considering that Israel can rapidly field an army of around 600,000 from a population of around 8 million. This means that, unless Putin decides to end the war, Ukraine’s troops will be pushed back again and again, losing soldiers in the process who cannot be replaced. Russia doesn’t even have to send its finest troops to achieve this — merely volunteer contract soldiers attracted by good pay, or Russian prisoners serving ordinary criminal sentences, recruited straight from their prison cells. Regardless of quality, though, the Russian army already outnumbers the Ukrainian, and the gap is becoming wider every day.

This arithmetic of this is inescapable: Nato countries will soon have to send soldiers to Ukraine, or else accept catastrophic defeat. The British and French, along with the Nordic countries, are already quietly preparing to send troops — both small elite units and logistics and support personnel — who can remain far from the front. The latter could play an essential role by releasing their Ukrainians counterparts for retraining in combat roles. Nato units could also relieve Ukrainians currently tied up in the recovery and repair of damaged equipment, and could take over the technical parts of existing training programmes for new recruits. These Nato soldiers might never see combat — but they don’t have to in order to help Ukraine make the most of its own scarce manpower.

Crucially, with China coming ever closer to an attack on Taiwan, the US cannot provide more troops than the roughly 40,000 who are already in Europe. Thus a momentous decision is in the post for the other Nato members, especially the most populous: Germany, France, Italy and Spain. If Europe cannot provide enough troops, Russia will prevail on the battlefield, and even if diplomacy successfully intervenes to avoid a complete debacle, Russian military power will have victoriously returned to Central Europe. At that point, Western European powers will have to rebuild their armed forces, whether they like it or not, starting with the return of compulsory military service. Perhaps in those circumstances, we might even witness an outbreak of nuclear nostalgia, foolishly harking back to the illusion that apocalyptic weaponry might be enough to keep the peace.

Most alarmingly, perhaps, Vladimir Putin has discussed the possibility of using nuclear weapons and his subordinate, the Chechen leader Ramzan Kadyrov, has called for the use of a “tactical” nuclear weapon on the battlefield in Ukraine. We wanted to assess how likely it is that a nuclear bomb might be used in the war. As well as that, what factors might increase the chance of that happening — if Ukraine continues to retake territory, or if Putin begins to lose his grip on power, will that increase or reduce the likelihood of a nuclear attack? Our forecasters were asked:

Will a nuclear weapon be detonated in Europe as an act of hostility before 30 April 2023?

Median forecast: 9%

That’s an extraordinarily high risk for a (just-over)-six-month period — if that level of danger was constant, there would be less than 20% chance of making it through any given decade without a nuclear explosion. The likelihood that the 75 years since the Second World War would have passed without any atomic bombs going off would be minuscule. But that group’s forecast hides a reasonable amount of variation. Some felt it was as low as 5%; others as high as 20%.

Forecasters were asked several questions about possible milestones in the war, such as Ukraine taking a particular city or region. How likely are those milestones, and if they happen, what will they say about the risk of a nuclear detonation? We asked four pairs of questions about the possible fall of four different cities: Sievierodonetsk, Melitopol, Chaplynka, and Mariupol. For each, we asked:

What is the probability that Ukraine’s armed forces will take control of the city before 1 April 2023?

If Ukraine’s armed forces do take control of the city before 1 April 2023, will a nuclear weapon be detonated in Europe as an act of hostility within the following 30 days?

If the risk of a nuclear weapon being detonated between now and 30 April 2023 is uniform over time, then our 9% forecast implies there is a rolling risk of 1.3% over a 30-day period for the foreseeable future. Here, we use 1.3% as our benchmark for examining the effect of each milestone.

All the forecasters felt that more cities were likely to return to Ukrainian hands. The aggregate forecast was 71% that Sievierodonetsk, just a few miles behind the Russian lines, would fall. One forecaster felt that its loss was “already factored in” to Russian military thinking. Melitopol, in the south-east, and Chaplynka, on the border of the Crimean peninsula in the south, were almost as likely — 48% and 44% likely, respectively.

Sievierodonetsk is a “test” of the risk of nuclear weapons, said one forecaster — it is likely to fall soon, and tactical nuclear weapons could easily be targeted at Ukrainian supply lines, and be used in a comparatively low-stakes way. But another felt that “if Russia was going to use nuclear weapons because of Sievierodonetsk, they probably should have already”. The likelihood of a nuclear weapon being detonated in Europe in the 30 days following the recapture of Sievierodonetsk, Melitopol, and Chaplynka is 3.3%, 5.1%, and 5.5% according to our forecasters. The implication here — with its risk elevated up to four times higher than our baseline monthly figure — is that a nuclear exchange is more likely to occur following exchanges in territorial control.

Mariupol, on the other hand, deep behind Russian lines and symbolically important after its long resistance, was deemed unlikely to fall, with an aggregate estimate of 21%. “In this case, it seems likely that the whole Russian lines have collapsed”, wrote one forecaster. “If Mariupol is taken, it probably means the war is completely lost for Russia, with the possible exception of Crimea”, wrote another.

Most of the forecasters felt that, if nuclear weapons were to be used, it would happen before Mariupol fell: “If they haven’t been used prior to this event, then I highly doubt this would be the turning point”, said one, although another sounded a note of caution: “If Mariupol is re-taken, it would be a devastating blow to Putin, so the risk of a hostile nuclear detonation after the re-capture of Mariupol wouldn’t be entirely eliminated.” Accordingly, if this takes place, our forecasters assign a 1.6% chance of a nuclear weapon being used over the subsequent 30 days — similar to our baseline rolling risk.

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Forecasters were also asked two pairs of questions about wider regions, the oblasts of Donetsk, Luhansk, Kherson, and Zaporizhia:

Will Russia be completely driven from Donetsk and Luhansk oblasts before 1 April 2023?

Will Russia lose all of Kherson and Zaporizhia oblasts before 1 April 2023?

What effect would the realisation of these scenarios have on the chance of a nuclear attack over the subsequent 30 days?

Driving Russia entirely out of the Kherson and Zaporizhia oblasts, in the south of Ukraine, seemed unlikely to most forecasters — with an aggregate forecast of 31% — but it would be a major humiliation if they were. “If Zaporizhia and Kherson fall, Putin is also likely to have few days left,” wrote one. “His troops would be thoroughly demoralised. I would suspect that he would want to make a desperate last stand at the border of Crimea.”

Whether it would increase the risk of nuclear weapons was unclear. “Again, this comes well after the point at which a tactical nuclear weapon ought to be used,” wrote one, although another felt that one might still be used “to deter any further Ukrainian gains in the (more important to Russia) territories of the Donbas and Crimea”. The aggregate forecast of the probability of a nuclear attack in the following month was 3.2%.

The loss of Donetsk and Luhansk oblasts — in the east of Ukraine, bordering Russia, filled with Russian-speaking ethnic Russians, and the parts that Russia initially occupied in February — would be especially damaging to Russia’s self-image. “The loss of them would be catastrophic for Putin,” wrote one forecaster, “because it would signal that he could not even hold territory where many of the locals may not oppose his forces”.

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Again, though, it was considered unlikely that both oblasts would be recaptured by Ukraine in the next six months, forecasted to be 18% likely. The risk of nuclear weapon usage 30 days after the realisation of this condition, 1.2%, was deemed to be lower than the baseline risk of 1.3%. “By this point”, according to one forecaster, “Russia will have essentially lost the war, and would have no incentive to launch a nuclear weapon.”

Turning toward the leadership of the countries directly involved in the conflict, we asked the forecasters whether either Vladimir Putin or Volodomyr Zelenskyy leaving office would affect the chance of a nuclear attack.

The chance of Zelenskyy leaving office was considered low — 5%, — without much variance. The only plausible reason the forecasters thought he would leave was if he was killed. If he was killed by a nuclear blast, that might make a second attack more likely, but in general it wasn’t thought that it would increase the risk: “If Zelenskyy dies then everything is up in the air,” said one. “Seems Putin no longer needs to resort to nuclear weapons.”

The chance of Putin leaving office, on the other hand, was thought to be higher — 10.5% — and with much greater variance, with one forecaster as high as 38%. If he leaves, it will not be in triumph. “If Vladimir Putin leaves office, it is likely due to a massive and humiliating defeat in Ukraine,” said one forecaster. It would also mean, they felt, that a nuclear detonation would be less likely. “The cases where Putin leaves office will almost universally involve the invasion being regarded as a complete failure,” said one, “and any replacement is almost certainly not going to escalate it”. They put the chance of a nuclear strike in the month after Putin leaves office at 2%.

A more complex scenario is one in which Putin faces a coup attempt. Forecasters were asked:

Will there be an attempted coup to remove Putin resulting in more than five arrests before 1 April 2023?

The forecasters again thought this to be unlikely but not vanishingly so: 15% (with significant variation — one put it at 33%). But they were unsure about whether it would raise or lower the risk of a nuclear blast: some felt it would make Putin edgy; others felt it would make him less willing to do anything that would further undermine his standing, such as starting a nuclear war.

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While the forecasters considered a broad range of factors that affect escalatory scenarios (from Russian military doctrine to the chance of nuclear launch orders being disobeyed), we have not identified any highly probable scenarios that turn the possibility of nuclear escalation into an inevitability. However, our forecasters clearly associate major losses in Russian-controlled territory with a higher chance of nuclear escalation. Ukrainian recapture of cities in the Kherson and Zaporizhia Oblasts, for instance, may coincide with increasingly desperate responses from Russia.

It’s important to note that this is only forecasting the chance of at least one bomb going off, somewhere in Europe. The obvious follow-up question would be: if one does, what are the likely outcomes? Will it lead to reprisals by NATO and the West on Russia? If so, will they be conventional or nuclear?

***

A version of this research first appeared on the Swift Centre.

But the answer is not so simple. Nuclear bombs have been dropped in conflict just once, in August 1945, by the United States. The horrific scenes of destruction and human suffering in Hiroshima and Nagasaki — and a death toll estimated to be about 200,000 civilians — provoked widespread condemnation. The theologian Reinhold Niebuhr wrote in the New York Times that the bombings were “morally indefensible”. Even hardened military figures were aghast. “We had adopted the ethical stand common to the barbarians in the Dark Ages,” grieved Admiral William Leahy, the US’s highest-ranking military commander.

Years later, the former Supreme Allied Commander Dwight Eisenhower told Newsweek: “It wasn’t necessary to hit them with that awful thing”. It was a comment that alluded to the military tactics behind the decision, which are separate from the more emotive philosophical debate over the morality of dropping the bomb. What does it mean for it to be “necessary” to drop a nuclear weapon? If it wasn’t necessary, then why did the US government — indeed, why would any government — willingly inflict such pain and suffering on civilians if there was a legitimate alternative path to peace?

The man who made the decision, Harry Truman, gave a lecture in 1959 to Columbia University students about the powers of the presidency. The talk was described as “earnest, good-humoured, and sometimes salty”, and was well-received by the 1,200-strong audience. But during the question and answer session, things took a more serious turn. A student thought it was odd that Truman had skirted over the nuclear question. When confronted with it, the usually good-natured Truman became a touch defensive:

“It was used in the war, and for your information, there were more people killed by the fire bombs in Tokyo than dropping of the atomic bombs accounted for. It was merely another powerful weapon in the arsenal of righteousness. The dropping of the bombs stopped the war, saved millions of lives”.

This is the orthodox view: dropping nuclear bombs saved lives. In this telling, the Japanese were not prepared to surrender, meaning the US and Soviet Union had two choices. They could either launch a land invasion, something on the scale of the D-Day landings in France, in which an estimated million US soldiers alone would be lost. Or they could prevent such a bloody undertaking by shocking the Japanese leadership into capitulation.

Death was not enough. The fire bombings of Tokyo, in which 100,000 people died in one night, had not proved sufficient. Something beyond conventional warfare was needed. Nuclear warfare was, as Truman’s Secretary of War Henry Stimson put it, the “least abhorrent choice”; though the bombs killed hundreds of thousands, even more would have died had they not been dropped.

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The orthodox account depends on a number of key claims, including that the Japanese were unwilling to surrender in the summer of 1945. These claims have since been challenged by “revisionist” historians, who argue that Japan was in fact on the verge of surrender when the bombs were dropped.

The previous summer, Japan had lost Saipan, an island in the South Pacific, to America, meaning its mainland was within range of USAF B-29 bombers. This made Japan highly vulnerable to sustained US attacks from the air. On the night of 9 March 1945, over 300 B-29 bombers descended on Tokyo, inflicting the deadliest bombing raid in world history. After that, the revisionist account goes, the question was not whether Japan would surrender but on what terms.

The revisionists place heavy emphasis on contemporary reports that suggest the Emperor and some (but not all) of his advisers were inclined to surrender, so long as his position could be guaranteed. A month before Hiroshima, Navy Secretary James Forrestal wrote in his diary: “The first real evidence of a Japanese desire to get out of the war came today… [indicating] the Emperor’s strong desire to secure a termination of the war”. A 1946 report by the US Strategic Bombing Survey concluded: “in all probability prior to 1 November 1945 [the planned invasion date], Japan would have surrendered even if the atomic bombs had not been dropped.”

The revisionists believe that the US rushed to use nuclear weapons not because of Japanese intransigence, but because of the Soviet Union. Throughout the war, the USSR and Japan had honoured a neutrality pact. Stalin, however, assured Truman’s predecessor Franklin Roosevelt that three months after victory in Europe was achieved, the Soviet Union would turn its attention to the Pacific and declare war on Japan. Truman and his closest advisers believed that Roosevelt had already conceded far too much to Stalin at Yalta, effectively giving the USSR free rein to dominate eastern Europe. They were not keen for the same thing to happen in Japan.

The first bomb was dropped on Hiroshima on August 6. On August 8, exactly three months after the surrender of Germany, the Soviet Union declared war on Japan. The following day, the second bomb was dropped on Nagasaki. The day after, the Japanese government offered its surrender. The revisionists look at this timeline and intuit that the nuclear bombs were dropped primarily for political rather than military reasons. The US wished to deny the USSR credit for ending the war, which would have given Stalin greater leverage in the post-war settlement.

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These two accounts — the orthodox and the revisionist — are the dominant explanations offered by historians, but they are not the only ones. Some scholars focus on the internal politics of Japan, arguing that the key figures who were ready for peace, not least the Emperor, were poor at communicating their wishes. They were, perhaps, drowned out by the powerful voices who wanted to continue the war. Many in the rank and file of the military were believed to be prepared to fight at all costs. US propaganda, which was racially inflected, tended to exploit the perception of Japan as a “warrior culture”, in which people would choose death over the shame of defeat. It was hard for Americans to discern Japan’s true intentions.

Other accounts suggest Truman was locked into decisions made by his predecessor. It was Roosevelt who had pushed for “unconditional surrender” at the Cairo conference in 1943. And the idea of dropping nuclear bombs on Japan was one Roosevelt had first dreamed up. In September 1944, he and Churchill met at FDR’s estate in Hyde Park, New York and agreed that “when a bomb is finally available, it might perhaps, after mutual consideration, be used against the Japanese, who should be warned that this bombardment will be repeated until they surrender”. Once Truman became president after Roosevelt’s death in April 1945, only one thing could apparently bring about “unconditional surrender”, and that was the bomb.

All of these interpretations offer important lessons to today’s leaders, but that last one is particularly chilling. If dropping the nuclear bomb was not militarily necessary to bring about an end to the war, then Truman opted for a decision that killed hundreds of thousands of civilians in part to save face. Once the US was committed to demanding “unconditional surrender” — for which there was enormous public support — Truman could hardly change his mind and say he wanted a negotiated peace. The new president, who had no political mandate of his own, would risk tanking his own domestic popularity before his term had even begun.

In other words, leaders must avoid making dangerous commitments from which either they or their successors will struggle to back down. Fortunately, in today’s conflict, no one has rashly insisted on “unconditional surrender”. A negotiated settlement is still possible.

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With that in mind, it’s worth considering the vital role of intermediaries in such high-powered conflicts. With the help of a neutral party, could the Japanese peace camp have communicated its position more clearly, so as to assist those within the US administration who might have urged caution? The Japanese saw the Soviets as possible intermediaries, a view that was shattered when Stalin declared war on them. Today, Israel seems to be the go-between, though the nation has been criticised by Western commentators for not condemning Putin.

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At Columbia University, Truman was asked, “With the development of nuclear weapons, do you think the United States would be willing to risk entering a third world war if a small European country in the Nato pact were to be involved in an act of aggression?” His answer was tepid: “I don’t know”. On consideration, he explained that the circumstances were now different, because unlike in 1945, the US was no longer the sole possessor of nuclear weapons: “I don’t think there’s any possible chance of doing what I tried to begin with, when we had absolute control of them, of nuclear weapons”.

This, strangely enough, is cause for optimism. When the US dropped its bombs on Japan, there was no other nuclear power to retaliate. In the intervening years, the threat of mutually assured destruction has seemingly prevented any other country from using such dangerous force.

It’s also worth observing that, while Ukrainians have shown an admirable fighting spirit in defending their nation from Russia, in the eyes of their people the two nations have a lot more in common than the Japanese did with the Americans. There has been no real attempt to use racial differences to demonise the enemy.

There is still a chance, then, that this war can be resolved through conventional methods. Putin is not in the same place Truman found himself in. Putin does not view it as “necessary” to drop a nuclear bomb. It has not yet become Putin’s “least abhorrent choice”. Let us hope it never does.