This sort of proliferation is often described as “horizontal.” The biggest immediate focus remains Iran, which has dramatically reduced the time it would require to develop one or more nuclear devices. An Iran with nuclear weapons might use them – or, even if not, might calculate that it could safely coerce or attack Israel or one or more of its Arab neighbors directly (or thorough one of its proxies) with non-nuclear, conventional weapons.
A nuclear-armed Iran would likely trigger a regional arms race. Several of its neighbors – particularly Saudi Arabia, Egypt, the United Arab Emirates, and Turkey – might well develop or acquire nuclear weapons of their own. Such a dynamic would further destabilize the world’s most troubled and volatile region.
But as important as this scenario is, another type of proliferation now merits attention: vertical proliferation, namely, increases in the quality and/or quantity of the nuclear arsenals of the nine countries that already possess these weapons. The danger is not only that nuclear weapons might be used in a war but also that the possibility of war would increase by emboldening governments – like Iran in the scenario above – to act more aggressively in pursuit of their geopolitical goals in the belief that they may act with impunity.
The fastest-growing nuclear arsenal in the world today belongs to China. It would appear that China believes that if it can match the US in this realm, it can deter the US from intervening on Taiwan’s behalf during any crisis over the island. China is on pace to catch up to the US and Russia in a decade – and is showing no interest either in participating in arms-control talks that would slow down its buildup or placing a ceiling on its capabilities.
Then there is North Korea. Neither economic sanctions nor diplomacy has succeeded in curtailing its nuclear program. North Korea is now thought to possess more than 50 warheads. Some are on missiles with intercontinental range and improving accuracy. Both China and Russia have assisted it, and further Russian assistance is likely given North Korea’s provision of weapons to Russia for use in Ukraine.
Again, the question is not only what North Korea might do with its nuclear arsenal. It is not farfetched to imagine a North Korean attack on South Korea or even Japan using conventional forces, coupled with a nuclear-backed threat to the US not to intervene. It is precisely this possibility that is fueling public pressure in South Korea to develop nuclear weapons, demonstrating that vertical proliferation can trigger horizontal proliferation, especially if countries currently protected by the US come to doubt America’s willingness to put itself at risk to defend them.
Russia offers another reason for worry. Russia and the US have the world’s two largest nuclear arsenals. Both are constrained by arms-control agreements (the New START Treaty) that limits the number of nuclear warheads that each can deploy to 1,550. (Additional warheads may be kept in storage, though.)
The agreement also limits how many launchers (planes, missiles, and submarines) carrying nuclear weapons can be fielded. And the pact also includes various arrangements to facilitate verification so that the two governments can be confident that the other is complying.
New START (ratified in 2011 and extended several times since) is due to expire in February 2026. Russia might well refuse to extend the treaty again, possibly because the performance of its armed forces in Ukraine has left it more dependent than ever on its nuclear arsenal. Or it may seek to barter its willingness to continue abiding by the agreement for US concessions on Ukraine.
What worries Washington is not only what Russia might do but also that the US now faces three adversaries with nuclear weapons who could coordinate their policies and pose a unified nuclear front in a crisis. All this is prompting the US to rethink its own nuclear posture.
In March, the US government reportedly completed its periodic review of its nuclear forces. At a minimum, billions of dollars will be spent on a new generation of bombers, missiles, and submarines. At worst, we could be entering an era of unstructured nuclear competition.
It all adds up to a dangerous moment. The taboo associated with nuclear weapons has grown weaker with time; few today were alive when the US used nuclear weapons twice against Japan to hasten World War II’s end. Indeed, Russian officials have hinted strongly at their readiness to use nuclear weapons in the context of the war in Ukraine.
Nuclear weapons played a stabilizing role during the Cold War. Arguably, their existence helped keep it cold. But there were only two decision-makers, and each had an inventory that could survive a first strike by the other, enabling it to retaliate in kind, thereby strengthening deterrence. And both sides mostly acted with a degree of caution, lest their competition escalate to direct conflict and precipitate a disastrous nuclear exchange.
Three and a half decades after the Cold War’s end, a new world is emerging, one characterized by nuclear arms races, potential new entrants into an ever less exclusive nuclear-weapons club, and a long list of deep disagreements over political arrangements in the Middle East, Europe, and Asia. This is not a situation that lends itself to a solution, but at best to effective management. One can only hope the leaders of this era will be up to the challenge.
