Even as Vladimir Putin’s Russia and Xi Jinping’s China have deepened their confrontation with the West over the past decade, they have remained aligned with the United States on at least one geopolitical project: dismantling or at least containing North Korea’s nuclear arsenal.

Until the Ukrainian war broke out two years ago.

In one of the most grim, Cold War-like throwbacks yet, Mr. Putin’s visit to Pyongyang on Wednesday to announce a “mutual assistance in case of aggression” agreement underscored how the effort by the world’s three largest nuclear powers to stop North Korea’s nuclear proliferation was already fraying. Mr. Putin and North Korean leader Kim Jong Un had just sounded its death knell.

Putin has done more than abandon any semblance of willingness to ensure North Korean nuclear restraint. He has pledged unspecified technical help that, if it includes several key technologies that Kim Jong Un is trying to perfect, could help North Korea design a warhead that can successfully re-enter the atmosphere, threatening many of its adversaries, starting with the United States.

Nowhere in Wednesday’s statement was there even a suggestion that North Korea should give up any of the 50 to 60 nuclear weapons it is estimated to possess. Instead, Putin declared that “Pyongyang has the right to take reasonable measures to strengthen its defense capabilities, ensure national security and protect sovereignty” — though he did not specify whether those measures would include further development of North Korea’s nuclear weapons.

While the shift is clear, what it might portend is still alarming. “There’s no doubt that this is a revival of Cold War-era security assurances,” said Victor Cha, who worked on North Korea issues during the George W. Bush administration. Those assurances date back to a now-defunct 1961 mutual defense treaty between Pyongyang and Moscow.

This time, however, the agreement “is based on transactional needs on both sides — Russia gets artillery shells, North Korea gets high-end military technology,” said Che, now at the Center for Strategic and International Studies. “They are not united by ideology as in the Cold War, but by their common opposition to the United States and the Western liberal order,” he added.

Cha said the new agreement will almost certainly solidify the increasingly formal security alliance between Japan, South Korea and the United States as threats from North Korea grow.

The Russians sent the signal 18 months ago.

Desperate for more shells to fuel the war in Ukraine, Putin turned to Kim Jong-un in late 2022 for some limited help with ammunition. The trickle has now reportedly become a torrent: Kim has provided 5 million rounds of ammunition, according to Western intelligence estimates, plus more North Korean-made ammunition stuffed into what the State Department calls 11,000 shipping containers. Then there are the ballistic missiles.

This reflects the fact that now, perhaps for the first time in its history, North Korea has a valuable bargaining chip that it needs in its confrontation with the West: it is a major weapons producer.

Initially, Kim was happy to receive oil and food in return. But in intelligence assessments circulating in Washington and Europe, officials say there is growing concern that the North Korean leader is now determined to overcome the last major technical hurdle to becoming a full-fledged nuclear weapons state — the ability to strike any city in the United States with a nuclear weapon.

Russia holds the keys; the question is whether it is willing to hand them over.

“Russia’s need for support on Ukraine has forced it to make long-sought concessions to China, North Korea and Iran,” Averell Haines, the U.S. director of national intelligence, told Congress in March. “This threatens to undermine long-standing nonproliferation norms.”

In her comments behind closed doors, she was far more specific, briefing key members of Congress on a range of techniques that Kim has yet to demonstrate he has mastered. Most of them involve keeping a nuclear warhead at an altitude of around 6,000 miles and ensuring it re-enters the atmosphere and hits its target.

It’s a development that many U.S. presidents have said is unacceptable. Before the Pyongyang summit this week, Cha wrote that the prospect of Russia helping North Korea “is the greatest threat to U.S. national security since the Korean War.”

“This relationship, which has deep historical roots and has been reinvigorated by the war in Ukraine, undermines security in Europe, Asia, and the American homeland,” he said. “The administration is courting disaster by relegating this issue to the back burner amid other important issues like the war in Ukraine and Gaza.”

Of course, Washington has faced so many warnings about the dangers of North Korea’s nuclear arsenal since it conducted its first nuclear test 18 years ago that they have become little more than background music to geopolitical turmoil.

Kim Jong-un has also shown a willingness to strike the United States in non-nuclear ways. Ten years ago, North Korea carried out a devastating hack on Sony Pictures, destroying much of its computing power. The incident was caused by Sony’s decision to release The Interview, a comedy starring Seth Rogen and James Franco, about two journalists who were sent to assassinate Kim Jong-un.

In many ways, it laid the foundation for modern cyber warfare, and by hacking into central banks and other lucrative Western targets, North Korea financed the expansion of its nuclear program.

A seemingly endless series of UN financial sanctions has failed to weaken North Korea’s nuclear expansion orThe closely related missile programThe U.S. sabotage efforts worked, but not for long.

The United States is therefore left to rely on a cold calculation of deterrence: long-range bomber exercises to remind North Korea that a strike against the United States or its allies would almost certainly result in its demise. But a credible security agreement with Moscow would complicate that reasoning. The terms of Wednesday’s deal, however, were not explicit.

Mr. Putin’s statement on Wednesday was also a reminder that North Korea’s continued success in developing nuclear weapons marks one of Washington’s greatest bipartisan failures, which began during the Clinton administration, when, in 1994, faced with a looming crisis with North Korea, the U.S. government considered destroying its nascent nuclear program before it could produce a single weapon.

President Clinton backed off, believing diplomacy was a better approach — beginning three decades of on-again, off-again negotiations. China and Russia helped, joining the “six-party talks” with North Korea that sought to buy off its nuclear program.

When the United Nations recently proposed extending the mandate of a monitoring panel set up by the United Nations to publicly present evidence of North Korea’s evasion of sanctions imposed after the collapse of the six-party talks, Russia successfully vetoed it, at least for now.

Now the United States, Japan, South Korea and other allies face two pressing challenges. The first is trying to prevent the transfer of technologies that are on Kim Jong-un’s shopping list, including ways to build quiet nuclear submarines and technology to circumvent missile defenses, according to Che and other experts.

U.S. intelligence officials have reported that Putin has provided North Korea with missile designs in the past, but there is little evidence that he helped the country develop actual nuclear weapons. Now, North Korea has leverage: Whether Kim Jong-un gets what he wants will determine whether North Korea continues to provide Putin with arms.

No one is watching this more closely than the Iranians. They are also supplying drones to the Russians. American officials believe the two sides are discussing missiles. Just last week, Iran ratcheted up the pressure on Israel and the United States, saying it was moving its most advanced centrifuges — capable of rapidly converting Iran’s fuel stockpile into the material needed to build three nuclear weapons — deep into an underground facility that may be beyond the reach of Israeli bunker-busting bombs.

If North Korea’s gambit works, the Iranians might also see the benefits of closer ties with Russia. Putin might conclude that he has nothing to lose anyway.