The smaller bombs that could turn Ukraine into a nuclear war zone

WASHINGTON (NYTIMES) - In destructive power, the behemoths of the Cold War dwarfed the US atomic bomb that destroyed Hiroshima. Washington's biggest test blast was 1,000 times as large. Moscow's was 3,000 times. On both sides, the idea was to deter strikes with threats of vast retaliation - with mutual assured destruction. The psychological bar was so high that nuclear strikes came to be seen as unthinkable.

Today, both Russia and the United States have nuclear arms that are much less destructive - their power just fractions of the Hiroshima bomb's force, their use perhaps less frightening and more thinkable.

Concern about these smaller arms has soared as Russian President Vladimir Putin, in the Ukraine war, has warned of his nuclear might, has put his atomic forces on alert and has had his military carry out risky attacks on nuclear power plants. The fear is that if Mr Putin feels cornered in the conflict, he might choose to detonate one of his lesser nuclear arms - breaking the taboo set 76 years ago after Hiroshima and Nagasaki.

Analysts note that Russian troops have long practised the transition from conventional to nuclear war, especially as a way to gain the upper hand after battlefield losses. And the military, they add, wielding the world's largest nuclear arsenal, has explored a variety of escalatory options that Mr Putin might choose from.

"The chances are low but rising," said nuclear expert Ulrich Kuhn from the University of Hamburg and the Carnegie Endowment for International Peace. "The war is not going well for the Russians and the pressure from the West is increasing," he observed.

Mr Putin might fire a weapon at an uninhabited area instead of at troops, Dr Kuhn said. In a 2018 study, he laid out a crisis scenario in which Moscow detonated a bomb over a remote part of the North Sea as a way to signal deadlier strikes to come.

"It feels horrible to talk about these things," Dr Kuhn said in an interview. "But we have to consider that this is becoming a possibility."

Washington expects more atomic moves from Mr Putin in the days ahead. Moscow is likely to "increasingly rely on its nuclear deterrent to signal the West and project strength" as the war and its consequences weaken Russia, Lieutenant-General Scott Berrier, director of the Defence Intelligence Agency, told the House Armed Services Committee on Thursday (March 17).

President Joe Biden is travelling to a Nato summit in Brussels this week to discuss the Russian invasion of Ukraine. The agenda is expected to include how the alliance will respond if Russia employs chemical, biological, cyber or nuclear weapons.

Mr James Clapper Jr, a retired Air Force general who served as president Barack Obama's director of national intelligence, said Moscow had lowered its bar for atomic use after the Cold War when the Russian army fell into disarray. Today, he added, Russia regards nuclear arms as utilitarian rather than unthinkable.

"They didn't care," Mr Clapper said of Russian troops' risking a radiation release earlier this month when they attacked the Zaporizhzhia nuclear reactor site - the largest not only in Ukraine but in Europe.

"They went ahead and fired on it. That's indicative of the Russian laissez-faire attitude. They don't make the distinctions that we do on nuclear weapons."

Mr Putin announced last month that he was putting Russian nuclear forces into "special combat readiness".

Dr Pavel Podvig, a longtime researcher of Russia's nuclear forces, said the alert had most likely primed the Russian command and control system for the possibility of receiving a nuclear order.

Dr Nina Tannenwald, a political scientist at Brown University who recently profiled the less powerful armaments, said that "Putin is using nuclear deterrence to have his way in Ukraine".

She added: "His nuclear weapons keep the West from intervening."

A global race for the smaller arms is intensifying. Though such weapons are less destructive by Cold War standards, modern estimates show that the equivalent of half a Hiroshima bomb, if detonated in midtown Manhattan, would kill or injure half a million people.

The case against these arms is that they undermine the nuclear taboo and make crisis situations even more dangerous. Their less destructive nature, critics say, can feed the illusion of atomic control when in fact their use can suddenly flare into a full-blown nuclear war.

A simulation devised by experts at Princeton University starts with Moscow firing a nuclear warning shot; Nato responds with a small strike, and the ensuing war yields more than 90 million casualties in its first few hours.

No arms control treaties regulate the lesser warheads, known sometimes as tactical or non-strategic nuclear weapons, so the nuclear superpowers make and deploy as many as they want. Russia has perhaps 2,000, according to Mr Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists, a private group in Washington. And the US has roughly 100 in Europe, a number limited by domestic policy disputes and the political complexities of basing them among Nato allies, whose populations often resist and protest the weapons' presence.

Over the years, the US and its Nato allies have sought to rival Russia's arsenal of lesser nuclear arms. It started decades ago as the US began sending bombs for fighter jets to military bases in Belgium, Germany, Italy, Turkey and the Netherlands. Dr Kuhn noted that the alliance, in contrast to Russia, does not conduct field drills practising a transition from conventional to nuclear war.

In 2010, Mr Obama, who had long advocated for a "nuclear-free world", decided to refurbish and improve the Nato weapons, turning them into smart bombs with manoeuverable fins that made their targeting highly precise. That, in turn, gave war planners the freedom to lower the weapons' variable explosive force to as little as 2 per cent of that of the Hiroshima bomb.

The reduced blast capability made breaking the nuclear taboo "more thinkable", General James Cartwright, a vice-chairman of the Joint Chiefs of Staff under Mr Obama, warned at the time. He nonetheless backed the programme because the high degree of precision lowered the risk of collateral damage and civilian casualties. But after years of funding and manufacturing delays, the refurbished bomb, known as the B61 Model 12, is not expected to be deployed in Europe until next year, Mr Kristensen said.

The steady Russian buildups and the slow US responses prompted the Donald Trump administration to propose a new missile warhead in 2018. Its destructive force was seen as roughly half that of the Hiroshima bomb, according to Mr Kristensen. It was to be deployed on the nation's fleet of 14 ballistic missile submarines.

While some experts warned that the bomb, known as the W76 Model 2, could make it more tempting for a president to order a nuclear strike, the Trump administration argued that the weapon would lower the risk of war by ensuring that Russia would face the threat of proportional counterstrikes. It was deployed in late 2019.

"It's all about psychology - deadly psychology," said Mr Franklin Miller, a nuclear expert who backed the new warhead and, before leaving public office in 2005, held Pentagon and White House posts for three decades. "If your opponent thinks he has a battlefield edge, you try to convince him that he's wrong."

When he was a candidate for the presidency, Mr Biden called the less powerful warhead a "bad idea" that would make presidents "more inclined" to use it. But Mr Kristensen said the Biden administration seemed unlikely to remove the new warhead from the nation's submarines.

It is unclear how Mr Biden would respond to the use of a nuclear weapon by Mr Putin.

A US response to a small Russian blast, experts say, might be to fire one of the new submarine-launched warheads into the wilds of Siberia or at a military base inside Russia. Mr Miller, the former government nuclear official and a former chairman of Nato's nuclear policy committee, said such a blast would be a way of signalling to Moscow that "this is serious, that things are getting out of hand".

Military strategists say a tit-for-tat rejoinder would throw the responsibility for further escalation back at Russia, making Moscow feel its ominous weight and ideally keeping the situation from spinning out of control despite the dangers in war of miscalculation and accident.

In a darker scenario, Mr Putin might resort to using atomic arms if the war in Ukraine spilled into neighbouring Nato states. All Nato members, including the US, are obliged to defend one another - potentially with salvos of nuclear warheads.