In June, Russian President Vladimir Putin and North Korean leader Kim Jong Un signed an agreement that pledges mutual aid if either country faces “aggression,” a strategic pact that comes as both face escalating standoffs with the West.

Experts said it could mark the strongest connection between Moscow and Pyongyang since the end of the Cold War. Both leaders described it as a major upgrade of their relations, covering security, trade, investment, cultural and humanitarian ties.

The summit came as Mr. Putin visited North Korea for the first time in 24 years and the U.S. and its allies expressed growing concerns over a possible arms arrangement in which Pyongyang provides Moscow with badly needed munitions for its war in Ukraine, in exchange for economic assistance and technology transfers that could enhance the threat posed by Mr. Kim’s nuclear weapons and missile program.

Mr. Kim met Mr. Putin at the airport, where the two shook hands, hugged twice and rode together in a limousine. The huge motorcade rolled through the capital’s brightly lit streets, where buildings were decorated with giant Russian flags and portraits of Mr. Putin.

After spending the night at a state guest house, Mr. Putin was welcomed in a ceremony at the city’s main square, filled with what appeared to be tens of thousands of spectators, including children with balloons and people in coordinated T-shirts of the red, white and blue national colors of both countries. Crowds lining the streets chanted “Welcome Putin” and waved flowers and flags.

During the visit, Mr. Kim said the two countries had a “fiery friendship,” and that the deal was their “strongest ever treaty,” putting the relationship at the level of an alliance. He vowed full support for Russia’s war in Ukraine. Mr. Putin called it a “breakthrough document” reflecting shared desires to move relations to a higher level.

Military Assistance

Just what the threshold for military assistance would be is currently unclear—maybe deliberately so, to deter other countries from putting it to the test. Mr. Putin initially told Russian media that the partnership provides for “mutual assistance in the event of aggression against one of the parties,” while North Korea’s state-run Korean Central News Agency later reported that the pact’s Article 4 called for assistance “in the event that either of the parties is invaded and pushed into a state of war.”

If the pact is triggered, the countries’ obligations are also vague, with KCNA reporting that if one is attacked the other must deploy “all means at its disposal without delay” to provide “military and other assistance.”

Experts note that the language is almost identical to that of a previous mutual defense pact from 1961 between the Soviet Union and North Korea, which was never put to the test.

Such pacts are not uncommon and are rarely invoked, while often being touted as a means of deterring aggression, though the agreement between the two unpredictable and autocratic leaders of nuclear nations immediately raised concerns globally.

The United States has many similar treaty obligations with other Asian nations, not to mention through NATO’s Article 5 provision, which says that an attack on a member of the alliance is to be considered an attack against all of its members. The only time NATO’s Article 5 has been invoked has been to come to the defense of the U.S. after the September 11, 2001, attacks.

Russia also has mutual defense pacts with several post-Soviet states through its Collective Security Treaty Organization alliance, including Belarus and Kazakhstan.

Nuclear Implications

North Korea is currently under heavy UN Security Council sanctions over its weapons program, while Russia also faces sanctions by the U.S. and its Western partners over its invasion of Ukraine.

U.S. and South Korean officials have accused the North of providing Russia with artillery, missiles and other military equipment for use in Ukraine, possibly in return for key military technologies and aid. A U.S. State Department spokesman said that in recent months, Washington has seen North Korea “unlawfully transfer dozens of ballistic missiles and over 11,000 containers of munitions to aid Russia’s war effort.”

Both Pyongyang and Moscow deny accusations of weapons transfers, which would violate multiple UN Security Council sanctions that Russia previously endorsed.

Along with China, Russia has provided political cover for Mr. Kim’s efforts to advance his nuclear arsenal, repeatedly blocking U.S.-led efforts to impose fresh UN sanctions on the North over its weapons tests.

In March, a Russian veto in the Security Council ended monitoring of UN sanctions against North Korea over its nuclear program, prompting Western accusations that Moscow is seeking to avoid scrutiny as it buys weapons from Pyongyang.

In Washington, U.S. Secretary of State Antony Blinken said Mr. Putin’s visit illustrates how Russia tries, “in desperation, to develop and to strengthen relations with countries that can provide it with what it needs to continue the war of aggression that it started against Ukraine.”

Sam Greene of the Center for European Policy Analysis said Mr. Putin’s trip to Pyongyang is an indication of how beholden he is to some other countries since invading Ukraine. Previously, “it was always the North Koreans coming to Russia. It wasn’t the other way around,” he said.

The trip is a good way to make “the West nervous” by demonstrating Moscow has interests and clout beyond Ukraine, Mr. Greene added.

The North could also seek to increase labor exports to Russia and other activities to get foreign currency in defiance of UN sanctions, according to the Institute for National Security Strategy, a think tank run by South Korea’s main spy agency. There will likely be talks about expanding cooperation in agriculture, fisheries and mining and further promoting Russian tourism to North Korea, the institute said.

Tensions on the Korean Peninsula are at their highest point in years, with the pace of both Mr. Kim’s weapons tests and combined military exercises involving the U.S., South Korea and Japan intensifying in a tit-for-tat cycle. The Koreas also have engaged in Cold War-style psychological warfare.

Limits

There are limits to the relationship, though, say analysts. Russia, the world’s biggest nuclear power, is not interested in major nuclear proliferation, especially by a neighbor. Nor is China, which despite growing clashes with Washington on foreign policy and trade issues is far from the international pariah that Russia and North Korea have become.

Both Moscow and Beijing, while ready to needle the United States and its Asian allies with public support for North Korea, have in the past criticized Pyongyang in public for its displays of missile or nuclear might.

Russia is due to outproduce all of the NATO military alliance on ammunition this year, so Mr. Putin’s reliance on Mr. Kim is limited.

And while the Soviet Union helped North Korea build a research reactor at Yongbyon, the Soviet-era Kremlin was always wary of the nuclear ambitions of Kim Il Sung—the state founder and Kim Jong Un’s grandfather—and found Pyongyang a provocative, difficult partner, especially when it tried to nudge Moscow by deepening ties with China.

Russia has also historically balanced its approach to the North with its relations with South Korea, a U.S. ally but economic powerhouse, said Anthony Rinna, a specialist in Korea-Russia relations at Sino-NK, a website that analyzes the region.

“By sidling up to North Korea, especially by upgrading their relationship to a comprehensive strategic partnership, Russia risks undermining its policy of diplomatic equidistance,” he said.

China’s Influence

With no obvious options, China appears to be keeping its distance as Russia and North Korea move closer to each other with the new defense pact, which could tilt the balance of power among the three authoritarian states.

Experts say China’s leaders are likely fretting over a potential loss of influence over North Korea after Mr. Kim and Mr. Putin signed the deal, and how that could increase instability on the Korean Peninsula.

Beijing may also be struggling to come up with a response to the Russia-North Korea partnership because it has conflicting goals: Keeping peace in the Koreas while countering the U.S. and its Western allies on the global stage.

Beijing so far has not commented on the deal, and only reiterated boilerplate statements that it seeks to uphold peace and stability on the Korean Peninsula and advance a political settlement of the North-South divide.

The Chinese response has been “very weak,” said Victor Cha, senior vice president for Asia and Korea chair at the Center for Strategic and International Studies, adding that it could be a sign that Beijing does not yet know what to do.

“Every option is a bad option,” he said. “You’re either unable to make a decision because of very strongly held competing views or...you’re just incapable of making a decision because you just don’t know how to evaluate the situation.”

Some in Beijing may welcome the Russia-North Korea partnership as a way of pushing back at America’s dominance in world affairs, but Mr. Cha said that “there is also a great deal of discomfort” in China, which does not want to lose its sway over its neighbor to Russia, does not want to see a destabilizing nuclear power on its doorstep, and does not want to bring the conflict in Europe to Asia.

But China is not raising these concerns publicly. “They don’t want to push Kim Jong Un further into the arms of Vladimir Putin,” Mr. Cha said, referring to the leaders of the two countries.

Lin Jian, a spokesperson for the Chinese Foreign Ministry, declined to comment on the new agreement. “The cooperation between Russia and the DPRK is a matter between two sovereign states. We do not have information on the relevant matter,” he said, referring to North Korea by the initials for its official name, the Democratic People’s Republic of Korea.

John Kirby, the White House national security spokesman, told reporters that the pact between Russia and North Korea “should be of concern to any country that believes that the U.N. Security Council resolutions ought to be abided by.” The Security Council has imposed sanctions on North Korea to try to stop its development of nuclear weapons.

Mr. Kirby also said the agreement “should be of concern to anybody who thinks that supporting the people of Ukraine is an important thing to do. And we would think that that concern would be shared by the People’s Republic of China.”

One area that China could be concerned about is whether Russia will help North Korea’s weapons program by sharing advanced technology, said Alexander Gabuev, director of the Carnegie Russia Eurasia Center.

“If China is indeed concerned, it has leverage in both Russia and North Korea and it could probably try to put some limitations to that relationship,” he said.

Complicated Relationships

The meeting between Mr. Putin and Mr. Kim was the latest chapter in decades of complicated political and military relationships in East Asia, where the Chinese Communist Party, once an underdog, has emerged as a leading power that wields influence over both North Korea and Russia.

That and other developments have raised alarms in the U.S. that Beijing, now the world’s second-largest economy, could challenge the U.S.-led world order by aligning itself with countries such as Russia, North Korea and Iran. Beijing has rejected that allegation.

Sun Yun, director of the China program at the Stimson Center, said Beijing does not want to form a three-way alliance with North Korea and Russia, because it “needs to keep its options open.”

Such a coalition could mean a new Cold War, something Beijing says it is determined to avoid, and locking itself to Pyongyang and Moscow would be contrary to China’s goals of maintaining relationships with Europe and improving ties with Japan and South Korea, she said.

Ms. Sun added that the rapprochement between North Korea and Moscow “opens up possibilities and potentials of uncertainty, but based on what has happened so far, I don’t think that China’s national interests have been undercut by this.”

Closer ties between Mr. Putin and Mr. Kim could weaken Beijing’s sway and leave it as the “biggest loser,” said Danny Russel, who was the top U.S. diplomat for Asia in the Obama administration.

“Apart from irritation over Putin’s intrusion into what most Chinese consider their sphere of influence, the real cost to China is that Russia’s embrace gives North Korea greater impunity and room to maneuver without consideration to Beijing’s interests,” he said.

Mr. Russel, now vice president for international security and diplomacy at the Asia Society Policy Institute, said that Mr. Kim is eager to reduce his country’s dependence on China.

“The dilution of Chinese leverage means Kim Jong Un can disregard Beijing’s calls for restraint,” he said, “and that is much more likely to create chaos at a time when [Chinese leader] Xi Jinping desperately wants stability.”

Keep your eyes on how Russia and North Korea’s partnership develops, and how this pact impacts their relationships with China, America and the rest of the world.

This article contains information from Reuters and The Associated Press.