US president-elect Donald Trump says the One China policy is "under negotiation". The Chinese are angry because it means that the US may recognise China and Taiwan as separate entities.
What is Trump's position on the 'One China' policy?
He has repeatedly said he will review the One China policy since winning the election in November.
The policy maintains that Taiwan and mainland China are one country, China.
The US has had only unofficial ties with Taiwan, the self-governing island that Beijing considers its territory. But the relationship between both nations has been ambiguous and contradictory. Washington sells weapons to Taiwan, despite failing to recognise it as an independent state.
Trump will be the first US president since 1979 to mention reviewing America's approach to the One China policy.
What is China's dispute with Taiwan?
China wants to completely absorb Taiwan into the mainland. Taiwan has been resisting.
The rift dates back to China's civil war in 1927, between the Communist Party of China led by Mao Zedong and the government of Nationalist Kuomintang (KMT).
The KMT was defeated but its leaders fled to Taiwan, which was still under the KMT's control. From Taiwan, the KMT continued to claim the entirety of China – just as the mainland claims Taiwan.
What was China's response to Trump?
Beijing said the One China policy is "non negotiable".
"The government of the People's Republic of China is the only legitimate government representing China," Beijing's foreign ministry spokesman Lu Kang said.
"That is the fact acknowledged by the international community and no one can change it."
Chinese media reported that Beijing would take "strong counter measures" against Trump's attempt to "impair" the One China policy; the Global Times called the incoming president's comments "despicable".
"If Trump is determined to use this gambit on taking office, a period of fierce, damaging interactions will be unavoidable, as Beijing will have no choice but to take off the gloves," the English-language China Daily said in an editorial.