Buffeted by the currents of diplomacy, South Korea is sometimes described as a "shrimp between two whales", and US president Donald Trump has touched nerves with remarks that the peninsula "used to be part of China".
The comments came after Trump hosted Chinese President Xi Jinping at his Mar-a-Lago retreat in Florida.
As the pair discussed ways to curb North Korea's nuclear ambitions amid mounting tensions, Trump told the Wall Street Journal, Xi "went into the history of China and Korea.
"And you know, you're talking about thousands of years... and many wars. And Korea actually used to be a part of China," Trump went on.
Beijing is Pyongyang's sole major ally and Washington wants it to do more about the North's nuclear and missile programmes, while the US has a security alliance with Seoul and stations more than 28,000 troops in South to defend it.
The exact details of what Xi said and whether Trump accurately represented him are not known, but South Koreans are wary of Chinese expansionism, and politicians, historians and citizens have been outraged.
The Korean peninsula has been heavily influenced by China politically and culturally for centuries.
But while its ruling kingdoms sometimes paid tribute to their giant neighbour, South Korean historians stress they did not come under its territorial control, despite repeated invasions.
Seoul's foreign ministry spokesman countered: "The fact that Korea was not part of China for thousands of years... is a clear historical fact acknowledged by the international community."
- 'Dumbfounded' -
Chinese foreign ministry spokesman Lu Kang declined to confirm the details of Xi's wording, saying that: "The Korean people should not be worried about it."
But Seoul's major JoongAng Ilbo daily said South Koreans had been "dumbfounded" by the Chinese leader.
"If Trump really conveyed Xi's words correctly, it is nothing but a grave challenge to the identity of the Korean people," it said.
South Korean historians and activists rallied outside the Chinese embassy on Friday to protest against Xi's "absurd remarks", Yonhap news agency reported.
Seoul and Beijing are at loggerheads over the deployment of the US missile defence system THAAD to the South to guard against threats from the North, infuriating China, which sees it as weakening its own capabilities.
Beijing has imposed a series of moves including a ban on group tours to the South and suspensions of South Korean businesses in China, which are seen by Seoul as economic retaliation.
South Korea's top-selling Chosun newspaper turned on both leaders, castigating Xi for his "premodern expansionist view" and Trump for a lack of awareness of diplomatic sensitivities.
"The fact that Trump publicly disclosed such a remark (by Xi) demonstrates that he is completely ignorant about the history of the Korean peninsula," it said in an editorial Friday.
- Relations 'at their lowest' -
The row came after South Koreans were dismayed by the revelation that the US aircraft carrier Carl Vinson and a supporting battle group were still thousands of kilometres (miles) from the peninsula, despite the White House saying it was dispatching them to the area as tensions mounted with the North.
Trump had described the vessels as an "armada" and "very powerful" and the revelation drew jeers and disappointment.
"What Mr Trump said was very important for the national security of South Korea," conservative South Korean presidential candidate Hong Joon-Pyo told the Wall Street Journal.
"If that was a lie, then during Trump's term, South Korea will not trust whatever Trump says."
The series of missteps demonstrate lack of communications within Trump's nascent administration, said Bong Young-Shik, analyst at the Yonsei University’s Institute for North Korean Studies.
"Trump is an outsider inexperienced with how Washington works... and the coordination among his White House, the State Department and the Pentagon seems to be poor for now," he told AFP.
Relations between Seoul and Beijing are "at their lowest for years", said Bong, and Trump's description of Xi's remark was likely to worsen them further, "whether he intended to or not".
Fears of potential Korean conflict have mounted in recent weeks with Pyongyang showing no sign of any willingness to abandon its nuclear and missile programmes, and the US saying that military action was an "option on the table".
The North has vast amounts of artillery deployed within striking range of Seoul, putting the South Korean capital at risk of devastation in even a conventional conflict.
The carrier controversy illustrated Trump's "unpredictable behaviour" that could lead to "disastrous events", the Kyunghyang daily said.
"We are worried whether the Trump administration... can properly handle a crisis on the Korean peninsula," it added. "The South Korean government should brace itself against the 'Trump Risk'."