"[If] their relationships are bad, then the whole structure in the region is equally weakened,” Brooks said, adding that a weaker structure invites Beijing to “create an alternative structure [favourable to] China”.

“If there's doubt in the countries on China's periphery about the commitment of the United States, there's no question in my mind that it benefits China,” he said.

The demise of GSOMIA, signed in 2016, would have undercut the efforts of the three democratic nations to deal with an increasingly assertive China and a wayward North Korea.

The US-led alliance with Japan and South Korea appeared in trouble after Seoul announced in August that it would exit GSOMIA amid a trade dispute and diplomatic row with Tokyo.

The impending break was fuelled by the legacy of Japan’s annexation and occupation of South Korea during the first half of the 20th century.

But the bilateral intelligence-sharing agreement was saved when Seoul, after a last-minute flurry of diplomacy, announced on Friday it would reverse its decision to exit the pact, just six hours before it would have expired.

Kim You-geun, a national security official at Seoul's presidential Blue House, confirmed that GSOMIA would not be allowed to lapse at midnight on Friday.

Despite the countries’ history, Japanese Prime Minister Shinzo Abe on Friday said that South Korea’s decision to remain in the pact, with conditions, was motivated by strategic considerations.

Seoul salvages military intelligence-sharing pact with Tokyo

“I stressed the importance of cooperation between Japan and South Korea, and Japan, South Korea and the United States,” Abe said.

Ryo Hinata-Yamaguchi, a visiting professor at Pusan National University in South Korea, said Seoul's decision sends a message that the US, South Korea and Japan are committed to working together, a declaration which may displease Beijing.

“The US will view that South Korea has restrained itself from undermining trilateral cooperation to effectively deal with North Korea, but also to prevent China and Russia from taking advantage of the weakening US alliance network,” Hinata-Yamaguchi said.

“This certainly discomforts China and Russia, who both strategically benefited from the weakening of the US alliance network,” the professor said.

US defence chief aims to save alliance that counters N Korea, China

Pyongyang will use the pact “as an excuse for its bellicose behaviour and make efforts to further stall dialogues, arguing that the US, Japan and South Korea are ganging up on North Korea,” Hinata-Yamaguchi said.

The pact’s revival comes with China and Russia growing closer as the unorthodox foreign policy of the administration of US President Donald Trump raises questions about the depth of the US commitment to the region and causes friction with traditional US allies.

General Mark Milley, chairman of the US Joint Chiefs of Staff, said in Tokyo last week that average Americans were growing sceptical about the need for a US troop presence in South Korea and Japan.

“Why are they needed there? How much does it cost? These are very rich and wealthy countries, why can't they defend themselves?” Milley asked.

Seoul fears US delaying envoy approval to punish it for ending pact

The Trump administration has insisted that South Korea and Japan quadruple their payments for US military deployments in their countries to roughly US$5 billion and US$8 billion, respectively.

Meanwhile, China and Russia seek to expand their political influence globally, a plan revealed amid their first joint long-range air patrol in the region in July.

Two Chinese H-6 bombers and two Russian Tu-95 strategic bombers performed a formation flight in the Sea of Japan – also known as the East Sea.

Then they moved on to the East China Sea and crossed the Air Defence Identification Zone of both South Korea and Japan, in a demonstration of the countries’ strengthening military cooperation.

Hinata-Yamaguchi said that South Korea and Japan still must resolve many challenges to fix their troubled relations.

“Both sides have recognised that nothing good has come out from the spats, yet they also faced major dilemmas in putting things back on track,” the professor said.

Japan suggests China is bigger threat than North Korea in defence review

“Japan and South Korea were negotiating until the last minute, indicating that there was much discussion about each other's demands and also procedures,” he said.

“The question is now about what concessions both sides are willing to make, and whether they will reach negotiations on not only issues relating to security, trade, and historical issues, but also ways to prevent future ruptures in relations.”

Timothy Heath, a former US Pacific Command analyst who is now a senior defence researcher at the Rand Corporation, said Seoul's decision to renew the pact is a “welcome move” for the stability of Northeast Asia and for US alliances and partnerships.

“The US does not have a China containment policy, but it does encourage cooperation between South Korea and Japan on intelligence and security matters to more effectively monitor Chinese military developments,” Heath said.

Reconciliation isn’t rocket science: why can’t South Korea and Japan get along?

“Evidence that Japan, South Korea and the United States are working together as allies should also strengthen deterrence against China and North Korea,” he said.

While China will see that the US is determined to strengthen its military alliances, it will also notice that the Japan-South Korea relationship remains fragile, according to Heath.

“I expect Beijing will look for more ways to encourage a breakdown in ROK-Japan security ties and to step up its criticism of US efforts to bolster its military commitments near China,” the analyst said.

“Russia has already tried to turn South Korea and Japan against each other with the flight of a military aircraft over islands disputed by Japan and South Korea. Moscow will likely conclude from Seoul's decision to extend the pact that the US is stepping up its efforts to strengthen its alliances in Asia,” Heath said.

“But I expect Russia to continue looking for ways to weaken America's alliances in Asia as well as in Europe.”

Can China take advantage of rift between South Korea and Japan?

Last year, China and Russia flexed their muscles during the Vostok 2018 war games in Russia’s far east.

In 2016, Chinese and Russian warships showed a similar coordinated move, sailing close to the disputed Senkaku islands – also known as the Diaoyudao – raising alarms for US allies in the region.

But Oriana Skylar Mastro, a resident scholar at the American Enterprise Institute specialising in Chinese military and security policy in Asia-Pacific, noted that “it becomes only problematic for Beijing if it implements aggressive measures in the region”.

“This primary focus of the intel-sharing is about maintaining awareness of the regional security environment … The main focus of the pact at this stage is maintaining readiness from North Korea’s missile provocations,” Mastro said.

Bruce Klingner, a former CIA Korea deputy chief, said that South Korea’s last-minute decision to remain in GSOMIA with Japan would provide an “opportunity for both nations, along with the United States, to exercise forward-looking prudent diplomacy”.

East Asia and the world would benefit if China, Japan and South Korea work as one

“A lot of crockery has been broken resulting in strained relations among Seoul, Tokyo and Washington,” Klingner, now a senior research fellow at the Heritage Foundation, said.

“South Korea and Japan must remedy their security and economic relations as well as seek to address long-standing animosity from the brutal 1910-1945 Japanese occupation.”

Additional reporting by Agence France-Presse