When China's leader, Xi Jinping, visited the United States' firm ally South Korea in 2014, it seemed to be the beginning of a promising courtship.
His host, President Park Geun-hye, returned the favour by going to Beijing last year for an important military parade that several other American allies boycotted, a gesture that Mr Xi may have believed could lead to weaning her away from Washington.
For her part, Ms Park hoped that her new friendship with Beijing -- South Korea's number one economic partner -- would tamp down the relentless pursuit of nuclear weapons by the leader of North Korea, Kim Jong-un.
But the prospect of a friendly new era between China and South Korea seemed to collapse this week. After North Korea, China's treaty ally, launched a rocket, apparently to test ballistic missile technology, South Korea embraced what China had been trying to prevent: an American anti-missile defence system that will be deployed on China's doorstep.
China now appears angrier at the South Koreans than at Mr Kim, who ignored its advice against the rocket launch.
Ms Park's government said it was entering talks with the Obama administration regarding the deployment of the American system, and the Pentagon said the installation, paid for by the United States, would take place as quickly as possible.
South Korea acted after China's response to the North's recent nuclear tests turned out to be more tepid than Ms Park had expected after nearly two years of Mr Xi's wooing, South Korean analysts say.
The system, known as THAAD, for Terminal High-Altitude Area Defence, would offer South Korea, and the nearly 30,000 US soldiers stationed there, superior protection against the North's growing nuclear challenge than Seoul's current inadequate missile defences, those analysts say.
"President Park was very disappointed and upset with Xi's inaction and silence against North Korea when she desperately needed Xi's help," said Kim Heung-kyu, director of the China Policy Institute at Ajou University in Suwon, South Korea. Mr Xi was then embarrassed domestically by Ms Park's rush to accept the American defence system, Mr Kim said.
"Xi Jinping's efforts to enlist President Park as a friend have not gone as well as he hoped," he said, "and she was certainly disappointed in his efforts to control Kim Jong-un."
After the rocket launch on Sunday, China expressed "regrets" and argued vigorously at the United Nations against sweeping new sanctions.
In contrast, China said it was "deeply concerned" about South Korea's decision to allow the deployment of the missile defence system. It warned that "every country must not undermine the security interest of other countries while pursuing its own security interests", clearly implying that the missile system was aimed at solidifying Washington's network of alliances in Northeast Asia rather than offering protection against North Korea.
To demonstrate its annoyance, the Chinese Foreign Ministry summoned the South Korean ambassador in Beijing, Kim Jang-soo, to protest the talks between Seoul and Washington on the missile defence system. In an apparent nod to even-handedness, the Chinese also called in the North Korean ambassador, Ji Jae-ryong, over the rocket launch.
China's anger at the imminence of an American missile system so close to its borders stems from two propositions, said Chu Shulong, a professor of international relations at Tsinghua University in Beijing.
First, many in the Chinese government do not believe that North Korea would use its nuclear weapons, Mr Chu said. Second, the belief that deploying the THAAD system is aimed principally at solidifying the United States' position in Northeast Asia is widespread in Beijing, where officials fear the ultimate goal is to contain China.
"North Korea is a bad regime, yes, everyone agrees on that," Mr Chu said. "Is North Korea going to use its weapons? Perhaps not. They are not seen as an immediate threat."
That judgement differs sharply from testimony to Congress on Tuesday by the United States' national intelligence director, James R Clapper Jr, who put North Korea at the top of what Washington views as nuclear- and proliferation-related threats.
Of more concern to the Chinese than the North's nuclear weapons, Mr Chu said, is the notion that THAAD would knit South Korea and Japan, two US allies that have their own deep squabbles, more tightly under an American umbrella.
"THAAD will bring South Korea and Japan closer to the US defence system, making much more of a military bloc that is targeting China and Russia," he said.
Seoul's decision to stand by its security guarantor in Washington comes as the overall relationship between the US and China has become more tense, marked not only by differences over North Korea but also by competition in the South China Sea.
Next week, President Barack Obama will host the 10 members of the Association of Southeast Asian Nations at Sunnylands, the estate in California where he and Mr Xi met in 2013. On that occasion, the two men pledged to work together on solving the North Korean nuclear question. This time, Mr Obama is likely to ask the participants, if only indirectly, to stand with Washington over Beijing.
For China, the "introduction of THAAD is a setback because it links South Korea to a US regional strategy", said Scott A Snyder, senior fellow for Korea studies at the Council on Foreign Relations. "It remains to be seen whether China will respond to this setback by further limiting cooperation with the United States on North Korea or whether China is able to impose costs on South Korea for its decision."