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It has now been more than 15 years — though it might feel a lot longer — since then-President Barack Obama gave a landmark speech in Prague pledging that the United States would take “concrete steps towards a world without nuclear weapons” and shift away from the Cold War mindset that placed these weapons at the center of the country’s national security strategy.

Though the rhetoric was bold — and would help award him a Nobel Peace Prize later that year — Obama’s vision was relatively conservative. He laid out no concrete steps toward that goal and conceded it might not happen in his lifetime, at a moment when the president was just 48.

But it undoubtedly fit the zeitgeist of the time. Even some of the staunchest Cold War hawks were arguing then that nuclear weapons were obsolete in a world where superpower competition was at a low ebb and leaders were more concerned about terrorists getting their hands on a loose nuke than a state deliberately using one. The administration was in the process of negotiating New START, a treaty under which both the US and Russia agreed to sweeping reductions in their nuclear arsenals.

In retrospect, however, Obama’s speech looks less like the dawn of a new era than an epitaph for a brief period of hope that was rapidly coming to an end. In a speech last month, Admiral Tony Radakin, the chief of the United Kingdom’s armed forces, said the world has entered a “third nuclear age.” The first was the Cold War, and the second was the post-Soviet period, which was “governed by disarmament efforts and counter-proliferation.” The third era — the one we’re in now — is defined by “the almost total absence of the security architectures that went before,” as Radakin put it.

This is an era in which both the US and Russia are spending heavily on new nuclear capabilities and China is rapidly building up its own arsenal; in which Russia is flagrantly using the threat of nuclear weapons as a tool of coercion in its ongoing war in Ukraine; in which North Korea has dozens of nukes and Iran is potentially close to developing one; and in which a number of US allies — unsure about the reliability of America’s security guarantees — are wondering whether they need a nuclear deterrent of their own.

It’s also one in which decades-old arms control agreements meant to limit nuclear proliferation are lapsing, with little momentum toward reviving or replacing them. Once the last of those agreements expires, there will be a real risk that the world could find itself in a new nuclear arms race — except instead of two nuclear rivals, as in the Cold War, there will be three, which is a far more destabilizing dynamic.

This is the era in which Donald Trump will be handed the nuclear codes on January 20, and with them the ability to launch a war that could wipe out human civilization. Trump, of course, has some experience with the job, including both nuclear diplomacy and brinksmanship. He also has an approach to foreign policy so erratic that it has prompted calls for more limits to be put on the president’s unilateral control over nuclear weapons use.

The global landscape has also changed significantly since the last time Trump took office. Ankit Panda, a nuclear security expert at the Carnegie Endowment for International Peace, put it simply: “The big change this time is that things are a lot worse.”

A more dangerous nuclear world

We can start with Russia’s invasion of Ukraine. While a nuclear weapon has not been detonated in the war, Russia has undoubtedly used the threat of them as a means to deter Ukraine’s international backers from intervening directly in the conflict. This has ranged from President Vladimir Putin’s frequent reminders about his nuclear arsenal — the world’s largest — to more concrete recent steps like lowering Russia’s official threshold for nuclear use and the recent use of the new hypersonic, nuclear-capable “Oreshnik” missile on the city of Dnipro.

How effective this saber-rattling has actually been is an open question. The risk of nuclear war is certainly one big reason why the US and its allies never seriously considered sending troops into Ukraine or imposing a no-fly zone over the country in the early days of the conflict. Either move would have risked putting the world’s two biggest nuclear powers in a direct military confrontation, possibly killing each other’s soldiers. But Washington’s gradual ramp-up in weapons aid and recent decisions to allow Ukraine to carry out long-range strikes on Russian territory has shown that the US is a lot less intimidated by Putin’s threats than it used to be.

Biden officials emphasize that this doesn’t mean there was never any reason for concern. According to journalist Bob Woodward, US intelligence officials believed at one point in 2022 that there was a 50 percent chance of Russia using a tactical nuke in Ukraine if the Russians were in danger of losing the southern Ukrainian city of Kherson. Russia did lose the city eventually, with no such nuclear response.

But there was no guarantee that’s how events would play out. In a recent interview, Mara Karlin, a former assistant secretary of defense, said that during the war, the world has been closer to nuclear use than it has been in........

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