Putin Once Tried to Curb North Korea’s Nuclear Program. That’s Now Over.

As Vladimir V. Putin’s Russia and Xi Jinping’s China deepened their confrontation with the West over the past decade, they were always united with the United States on at least one geopolitical project: dismantling or at least containing North Korea’s nuclear arsenal.

That is, until the war in Ukraine broke out two years ago.

In one of the starkest back-to-the-Cold War moments yet, Mr. Putin’s visit Wednesday to Pyongyang — and the announcement of a pact to provide “mutual assistance in the event of aggression” — underscored that efforts by the world’s three biggest nuclear powers to halt nuclear proliferation by North Korea had been dying for some time. Mr. Putin and Kim Jong-un, the North’s leader, just presided over the memorial service.

Mr. Putin did far more than drop any semblance of a desire to ensure nuclear restraint. He promised unspecified technological help that — if it includes the few critical technologies Mr. Kim has sought to perfect — could help the North design a warhead that could survive re-entry into the atmosphere and threaten its many adversaries, starting with the United States.

Nowhere in the statements made Wednesday was there even a hint that North Korea should give up any of its estimated 50 or 60 nuclear weapons. To the contrary, Mr. Putin declared: “Pyongyang has the right to take reasonable measures to strengthen its own defense capability, ensure national security and protect sovereignty” — though he did not address whether those measures included further developing the North’s nuclear weapons.

While the shift has been clear-cut, what it could portend is stunning. “This is a renewal of Cold War-era security guarantees, no doubt,” said Victor Cha, who worked on North Korea issues during the George W. Bush administration. Those guarantees date to a now-defunct 1961 mutual defense treaty between Pyongyang and Moscow.

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