The turning point came in early July, when Park, the Putonghua-speaking South Korean president whom Xi described as “an old friend of the Chinese people”, agreed to the deployment of a powerful US anti-missile system on Korean soil.

To Beijing, Park’s decision marks a severe setback for China’s foreign policy efforts on the Korean Peninsula after years of attempts to draw South Korea away from its long-time ally, the US, and exert leverage on Japan, another firm ally of the US.

Analysts say Park’s decision changed the geopolitical situation on the divided Korean Peninsula overnight and presented Beijing with a strategic nightmare, turning the peninsula from a shield at its doorstep to a spear. They said Beijing was now faced with an increasingly unpredictable North Korean regime in Pyongyang and a strengthened Seoul-Washington alliance, with concerns only heightened earlier this month when Seoul hinted that it might share military intelligence with Tokyo.

Chinese foreign ministry officials warned Beijing would take all necessary measures to safeguard its interests.

Without directly mentioning Park, People’s Daily, the Communist Party’s mouthpiece newspaper, said in an editorial that her agreement to the deployment of the Terminal High Altitude Area Defence (THAAD) system was part of “America’s strategic plot”.

“(She) is well aware of the real direction of the THAAD anti-missile system,” it said. “(She) has no hesitation about undermining regional stability and flagrantly damaging the security interests of neighbouring powers,”

On August 8, Park broke her silence following weeks of strong criticism from Beijing. In a hardline response to a visit to Beijing by six South Korean opposition lawmakers, she said they were “sympathising with China” and accused opponents of the THAAD deployment of making “absurd arguments that jibe with North Korea’s”. Her governing Saenuri Party labelled the opposition lawmakers “flunkies of China”.

A day earlier, Park’s senior press secretary, Kim Sung-woo, lashed out at Beijing’s intensified propaganda campaign attacking the planned deployment.

“Rather than taking issue with our purely defensive action, China should raise issues in a stronger manner with North Korea, which is breaking peace and stability on the Korean Peninsula and Northeast Asia by conducting four nuclear tests and, just this year, launching more than 10 ballistic missiles,” Kim said.

Analysts from both South Korea and China say the recent diplomatic standoff between Seoul and Beijing has put “an end to the Xi-Park honeymoon period”.

“They are like a couple who fell in love with each other quickly, but they didn’t speak the other’s language and they didn’t understand what the other meant,” said Lee Seong-Hyon, a research fellow at the Sejong Institute in Seoul.

There is no doubt that Park, the daughter of former South Korean president Park Chung-hee, is better placed than most to be a popular foreign leader in China.

She and Xi shared similar experiences when they were young. In her biography, Park said she gained inner peace following the assassination of her parents in 1979 by reading The History of Chinese Philosophy, a book by Chinese scholar Feng Youlan. That same year, Xi finished his studies at Tsinghua University after spending six years in a village in rural Hebei province during the Cultural Revolution, a time when his father was persecuted, jailed and spent long periods in confinement.

Park, who is not married, is well known for her passion for Chinese culture and once said that Zhao Zilong, a general in the Three Kingdoms period, was her first love.

When she succeeded the pro-US Lee Myung-bak in 2013, Park took immediate steps to bolster ties with Beijing. In her inauguration speech on the first day of presidency she said diplomacy with China was a top priority.

That could explain why Beijing gave her an exceptionally enthusiastic reception when she chose Beijing, and not Tokyo, like her predecessors, as the destination of her second international visit as president in June 2013. She made the first, to the US, a month earlier.

She was invited to have a “special lunch” with Xi following a summit meeting and state dinner, and Xi presented her a calligraphic work as souvenir. It was a famous sentence from a Tang Dynasty poem that read “Ascend another storey to see a thousand miles”, and was an indication of his high hopes for an improvement in bilateral ties.

A year later, Xi chose to go to Seoul rather than Pyongyang on his first state visit to the peninsula, a move analysts saw as a sign that Beijing was shifting away from Pyongyang, its traditional ally, and closer to Seoul.

Bilateral relations reached to a new peak last year. In March, Seoul joined the Chinese-led Asian Infrastructure Investment Bank as a founding member. Three months later, Beijing and Seoul signed a landmark free-trade agreement. And three months after that Park was the only leader of a US ally to stand alongside Xi at a military parade in Beijing commemorating the 70th anniversary of victory over Japan.

Analysts said South Korea’s recent decision to turn back to the US highlighted the limited political trust between Seoul and Beijing, with Pyongyang’s fourth nuclear test, in January, being the final straw.

“Frequent meetings between the two leaders have not evolved into an institutional relationship,” said Woo Jung-Yeop, a research fellow at the Asan Institute for Policy Studies in Seoul. “When it comes to substantial issues including issues related to North Korea, their views and policies have diverged.

“For example, after North Korea’s nuclear test, it took the two leaders a month to talk over the so called hotline. The hotline between two leaders turned out to be not so hot when dealing with North Korea issues.”

Sun Xingjie, a Korean affairs expert at Jilin University, said the diplomatic standoff reflected Park’s deep disappointment with Beijing after years of economic courtship.

“The bigger the expectations, the bigger the disappointment,” Sun said. “When Park found that China failed to keep a rein on Pyongyang, unrealistic hopes were broken and feeling deeply insecure, she turned to the US instead.”

Beijing has reacted by banning new shows with South Korean stars from Chinese television screens. But Seoul is more concerned about the risk of further economic retaliation from China – its biggest trade partner – especially as South Korea’s exports have been falling for 19 months amid a global economic slowdown.

Analysts warned that a long-term standoff between Seoul and Beijing would benefit neither side.

Woo said that for South Korea, such confrontations would give North Korea a chance to “bring the Northeast Asian order back to the cold war period when China and Russia backed North Korea and did not have good relations with South Korea and Japan”.

Sun said a new cold war would push China into a corner and compromise its interests on the Korean Peninsula if South Korea drifted into the orbit of the US and Japan.

Since both governments are unwilling to give ground, it is hard to predict smiles at a Xi-Park reunion at next month’s summit.

Analysts said that in the long term, there was a need for talks between the two leaders to address the issue of North Korea.

But in the meantime, Woo said, “it is likely that the two leaders will end up reiterating their prior stances and core principles [at the G20 summit].”

Sino-Korean relations timeline

1950 – China supports North Korea in the Korean war

1972 – South Korea issues “June 23 Announcement”, altering its policy to allow trade with most communist countries

1974 – South Korea lifts its ban on postal exchanges with communist countries

1979 – China and South Korea begin indirect trade

1983 – A hijacked Chinese airliner lands in South Korea. Thirty-three Chinese officials travel to Seoul to negotiate its return

1992 – South Korea and the People’s Republic of China establish diplomatic relations

South Korean President Roh Tae-woo becomes the country’s first head of state to visit China

1998 – The Sino-South Korean relationship is upgraded to “cooperative partnership”

2002 – The bilateral relationship is further upgraded to “comprehensive cooperative partnership”

2004 – Presidents Hu Jintao and Roh-Moo-hyun declare the launch of an unofficial study into the feasibility of a China-South Korea free-trade area.

2008 – The relationship between the two countries is upgraded to a “strategic cooperative partnership”

2013 – South Korean President Park Geun-hye visits China, rather than Japan, in second overseas trip as head of state

2014 – Xi Jinping becomes the first Chinese leader to visit South Korea before visiting North Korea

2015 – South Korea and China sign free-trade agreement