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: preventing North Korea’s nuclear arsenal from growing, or becoming more accurate.

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 better target 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. He was referring 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, he said, adding: “They are united not by ideology, as in the Cold War, but in common opposition to the U.S. and the Western liberal order.”