Last week New START, the last of the Cold War-era nuclear arms control agreements between Moscow and Washington, expired. The world now no longer has any legal security infrastructure with which to limit its most dangerous weapons. Has the starting gun been fired on a new, global nuclear arms race?

As the clock ticked towards expiry, Pope Leo called on Washington and Moscow not to abandon the limits set in the treaty. ‘I issue an urgent appeal not to let this instrument lapse’, Leo said at his weekly audience. ‘It is more urgent than ever to replace the logic of fear and distrust with a shared ethic capable of guiding choices toward the common good.’ Daryl Kimball, the executive director of the Arms Control Association, warned that ‘we could see a dangerous three-way arms race’ between Russia, the United States and China.

But in many ways the START agreements, first signed in July 1991 between George H.W. Bush and Mikhail Gorbachev, were a pact for a different age. Ever since 1952 when ‘Ivy Mike’, the world’s first hydrogen bomb, completely vaporised the island of Elugelab with a detonation 500 times more powerful than the bomb dropped on Nagasaki, mankind has possessed the power to extinguish itself as a species. The doctrine of Mutually Assured Destruction (MAD) was born. Starting from the early 1960s, the Soviet Union and the US, driven by mutual paranoia, built up ever larger arsenals of warheads that reached a peak of more than 70,000 in 1986. The end of the superpowers’ endless escalation of their nuclear capacity under Mikhail Gorbachev and Ronald Reagan marked a return to sanity – as well as the true end of the Cold War.

Today, nuclear weapons remain as a deadly danger to the security of the planet, but three fundamental things have changed about the nature of the threat. First, until the 1980s both superpowers sought superiority through massive numbers of missiles, trusting in volume to overcome whatever defences the enemy could offer. Today, the focus has shifted to unstoppable missiles traveling at hypersonic speeds to accurately destroy specific targets.

The superpowers also vied to build ever larger bombs. The so-called Tsar Bomb detonated over the White Sea on 30 October 1961 generating a 56-megaton blast over 2,000 times more powerful than Hiroshima. The blast blew windows 900 kilometres away in Finland. Now, the clear danger is from miniaturised, so-called ‘tactical’ nuclear weapons designed for battlefield use. The rulebook on the deployment of tactical nukes has yet to be written.

Third, the number of nuclear-armed powers has grown from three when START was signed (with China a mere minnow in nuclear terms) to five declared (US, Russia, United Kingdom, France and China) and four undeclared powers – India, Pakistan, North Korea, and Israel. Iran is classed as a ‘threshold state’ capable of uranium enrichment and rapid advance to a usable bomb, while Japan and South Korea possess the technical expertise and material capacity to produce weapons quickly. Russia, Iran, India and Pakistan are currently in, or have recently been in, a state of war with their regional neighbours.

In that sense New START, with its focus on controlling numbers of deployed warheads backed up with a regime of mutual US-Russian inspections, is no longer relevant – both because China was never a signatory and also because the underlying logic of START failed to address the more urgent dangers of proliferation and tactical nuclear use. As Donald Trump pointed out, today’s nuclear escalation story is no longer about increasing the numbers of warheads. ‘If there’s ever a time when we need nuclear weapons like the kind of weapons that we’re building and that Russia has and that China has to a lesser extent but will have, that’s going to be a very sad day,’ Trump said in February last year. ‘That’s going to be probably oblivion.’

A treaty capping numbers of warheads and delivery systems does not, in itself, limit potential use. As for the risk of accidental alarms and unintended provocations, a raft of practical measures remains in place – from the US-Russia Hotline (1963), Non-Proliferation Treaty (1970), Accident Measures (1971), Incidents at Sea INCSEA (1972), Prevention of Nuclear War (1973), Nuclear Risk Reduction (1987), Ballistic Missile Notification (1988) and Dangerous Measures (1989). Furthermore, for most of the Cold War it was not treaties but clear deterrence relationships that prevented nuclear war – which made for a stable world if not exactly a safe one.

Today, the challenge for Washington is that the old Cold War balance of nuclear deterrence is shifting fast. China is rapidly building a brand-new arsenal while Russia devotes huge resources to developing new delivery systems and warheads. In many ways the new nuclear arms race began years before the expiry of START – and it’s a race that the US is losing.

In the 13 years that Xi Jinping has been in power the number of warheads at Beijing’s disposal has risen from under 250 to over 600 today. That’s small beer compared to the US and Russia’s estimated combined total of over 11,000 (though under START, each side was allowed only 1550 ‘deployed’, in other words battle-ready, warheads). China also has fully peer-comparable nuclear capacity across the ‘nuclear triad’ of land, sea and air-launched missiles. Beijing is building a new generation of silent ballistic missile submarines – the Type 096 – which are as hard to track as US and Russian ‘boomers’. A pair of fast-breeder reactors, built specifically to create highly enriched nuclear warhead fuel, have come online at Xiapu, opposite Taiwan. According to US Admiral Charles Richard, Commander of US Strategic Command, ‘the explosive growth and modernisation of [China’s] nuclear and conventional forces can only be what I describe as breathtaking’. As for China’s capabilities versus America’s, ‘as I assess our level of deterrence … the ship is slowly sinking. It is sinking slowly, but it is sinking, as fundamentally they are putting capability in the field faster than we are.’

It is Russia, however, that has made the greatest leaps in the two most dangerous areas of nuclear weapons – ultra-fast missiles and tactical nukes. When Vladimir Putin announced his new-generation hypersonic Oreshnik missile during his 2018 state-of-the-nation speech, he used a deliberately provocative video of the weapons flying over the US. In his missile assaults on Ukraine Putin’s forces have used Oreshniks – despite their staggering $30 million cost per missile – with multiple conventional warheads. Smaller Kinzhal and Zircon missiles have also been used more frequently to demonstrate their invincibility to missile defences. Last year, Putin further alarmed the world by presenting the Poseidon, a supposedly unstoppable nuclear-powered torpedo that can travel in deep oceans for years without refuelling.

Russia is now developing an even faster ballistic missile, the Mach-27 Avangard, while the Chinese are working on the DF-41, which travels 25 times the speed of sound – fast enough to fly from Eurasia to the US heartland in less than half an hour. While Moscow and Beijing have developed two generations of hypersonic missiles the US has none. A planned ‘Dark Eagle’, programme is under way – but that reportedly is slower than the Oreshnik with a top speed of Mach 5.

It’s a similar story with tactical nuclear weapons, which Russia has been actively developing for at least a decade. Typically, so-called ‘tactical’ nuclear weapons have a yield of single-figure kilotons – smaller than the first atomic bomb detonated over Hiroshima, which had an explosive power of 20,000 tons of TNT and killed over 70,000 people, but not by much. Russia has developed short-range ballistic and cruise missiles fired from dual-use bombers and fighter-bombers, as well as nuclear cruise missiles and torpedoes and warheads for naval platforms. Reportedly, Moscow’s tactical arsenal is around 1,500 such weapons. And, crucially, these systems are designed to be mobile, survivable and integrated into conventional army units – reflecting a doctrine that treats tactical nuclear weapons as part of a combined arms approach.

By contrast, the US, officially at least, has just a thousand tactical nukes in the form of simple gravity bombs deployed in Europe. A new generation of US nuclear-armed sea-launched cruise missiles has been debated for years and remains on the Pentagon’s programme of record, though its exact development timeline and deployment remains under discussion.

Not only is Russia actively producing tactical nukes, but, unlike the US, the Kremlin’s doctrine has openly signalled that tactical nuclear weapons play a role in its conventional war-fighting doctrine. Indeed, in late 2024 Putin revised Russia’s nuclear doctrine to expand the circumstances under which nuclear use might be considered. According to the revised Fundamentals of State Policy, the Kremlin would count ‘aggression by a non-nuclear state supported by a nuclear power’ as a joint attack on Russia. Also, a conventional attack that ‘threatens the sovereignty or territorial integrity’ of Russia could also trigger a nuclear response – a significant shift from the old policy that ruled out nuclear war in all circumstances but an ‘existential danger’ to the state.

China, by contrast, has a strict ‘No First Use’ policy that rules out nuclear weapons use except in retaliation against nuclear attacks. The US stands somewhere in between, neither ruling out first use but also restricting nuclear strikes to ‘extreme circumstances to defend vital interests’.

Ultimately, however, the scenario that keeps diplomats awake at night is not the prospect of Beijing, Washington or Moscow going to war but the rising threat of proliferation to smaller countries in volatile regions. What is the shakiest – and most recent – part of the unraveling of the old world nuclear order is a new fear that the US is unwilling to protect its old Cold War allies with the American nuclear umbrella. The implicit bargain that smaller countries would forbear from developing nuclear weapons as long as Washington’s security guarantees remained in place. For the South Koreans and Japanese that certainly is looking less ironclad. As a result, debates about Seoul’s acquisition of an independent nuclear deterrent have moved from the margins toward the mainstream. In Japan, victim of the only nuclear attacks in history, the taboo on nuclear arms runs deeper – but the prospect is being publicly aired. Prime Minister Sanae Takaichi hinted in December at a possible revision of the country’s longstanding non-nuclear policies, which prohibit the introduction of nuclear weapons into Japan’s territory.

It’s this global threat of proliferation, rather than the practicalities of capping strategic warhead numbers, that makes reforming a top-level entente between the world’s largest nuclear powers especially important. Ironically, for the last two decades it’s actually been Russia that has shown most willing to revamp Cold War arms control agreements. It was President George W. Bush who began the great unraveling of detente in 2002 when he unilaterally withdrew the US from the Anti-Ballistic Missile Treaty (ABM), first signed between Leonid Brezhnev and Richard Nixon in 1972. ABM forbade either side from developing defensive systems to counter ballistic missile attack – thereby paradoxically preserving peace by ensuring that both sides were equally vulnerable. Vladimir Putin, just two years into his first term, was horrified by what the Kremlin saw as US perfidy and irresponsibility. The collapse of ABM was followed by a series of people-power democratic revolutions across the former Soviet Union, which Putin saw as US-orchestrated aggression, and then by NATO expansion. A profound paranoia and distrust of Washington was born.

Nonetheless, when President Joe Biden suggested renewing START in 2021 the Kremlin agreed – and also offered to talk about restoring the 1987 Intermediate-Range Nuclear Forces Treaty (INF), which Trump pulled out of in 2019, alleging Russian non-compliance. Even as the deadline to the expiration of New START ticked down earlier this month the Russian Foreign Ministry offered to negotiate a last-minute renewal – only to receive no response from the White House. ‘No message is a message in itself.’ As Russian Deputy Foreign Minister Sergei Ryabkov put it: ‘Russia is ready for a new world with no nuclear limits.’

That is a bluff. In truth, the Kremlin has indicated that it would like to get a new architecture of international security guarantees in place – but only as part of a peace deal on Ukraine. On a practical level it’s significant that US General Alexus Grynkewich, the commander of the United States European Command and Supreme Allied Commander Europe, joined recent Russia-US-Ukraine peace talks in Abu Dhabi. Grynkewich and his Russian counterparts agreed to resume top-level military contacts between the US and Russian general staffs. That’s reassuring, but still a very long way from a new arms control deal.

The immediate stumbling block is China. Trump tried and failed to strike a three-way nuclear arms limitation deal that included Beijing in his first term. Now he is trying again. ‘Obviously, the president’s been clear in the past that in order to have true arms control in the 21st century, it’s impossible to do something that doesn’t include China because of their vast and rapidly growing stockpile’, US Secretary of State Marco Rubio said last week. But Beijing remains reluctant. ‘China’s nuclear forces are not at all on the same scale as those of the US and Russia’, said Foreign Ministry spokesperson Lin Jian. ‘Thus China will not participate in nuclear disarmament negotiations at the current stage.’

Of the world’s three nuclear superpowers, only America is keen to cut down nuclear stockpiles. ‘I want to… cut our military budget in half,’ Trump said at the outset of his second term. ‘There’s no reason for us to be building brand-new nuclear weapons; we already have so many.’ For Russia, its nuclear arsenal – along with its permanent seat on the United Nations’ Security Council – are key legacies of a great-power status that it has otherwise lost in terms of economic and soft power. By the same logic, a world-class system of nuclear weapons is an essential part of China’s rise to true superpower status.

In that sense, the ghosts of the Cold War are still with us. Even as direct ideological and military confrontation has been replaced by economic rivalry – and what Russia regards as an indirect proxy war being waged by the West in Ukraine – the preservation of massive, destructive nuclear potential remains the most weighty currency of international politics.

As a result, the world has, largely unnoticed, drifted away from cooperative deterrence towards competitive rearmament on the part of America’s rivals. Agreed programmes of verification have been replaced by mutual suspicion of the other side’s intentions. ‘Without facts, worst-case thinking takes over,’ said Michael McFaul, President Barack Obama’s Russian policy chief. ‘And that’s dangerous for nuclear powers.’ A relatively balanced system that may have failed to prevent dozens of proxy wars but was at least successful in preventing major nuclear confrontation has crumbled into history. For the first time since the Cuban Missile Crisis, the use of nuclear weapons has ceased to be unthinkable.