One phone call between President-elect Donald Trump and Taiwan's leader risks souring US relations with China, the largest holder of US government debt.
What was the phone call about?
Trump said Taiwanese President Tsai Ing-wen called to congratulate him on his election win. The conversation was the first in 37 years between the leaders of both nations after Washington endorsed the "One China" policy that sees Taiwan as part of China.
The call has led to speculation that Washington's position on Taiwan could change.
Taiwan said that the call was pre-arranged and that both leaders spoke about strengthening ties and working closer. Trump's transition team said the conversation focused on the "close economic, political and security ties that exist between Taiwan and the US".
Why is China so worried?
Tsai, Taiwan's first woman president, who took office in May, has challenged the policy that Taiwan is part of China.
Beijing is also concerned that Tsai may aim to meet with Trump's transition team when she is in New York in January.
Trump has not indicated if official policy on Taiwan will change but he defended speaking to Tsai.
Interesting how the U.S. sells Taiwan billions of dollars of military equipment but I should not accept a congratulatory call.— Donald J. Trump (@realDonaldTrump) December 3, 2016
What is the US policy on Taiwan?
Under former President Jimmy Carter, the US cut diplomatic relations with Taiwan in 1979 and recognised it as part of China.
Trump's conversation with Tsai last week was the first official contact between the US and Taiwan.
But the relationship between both nations has been ambiguous and at times contradictory. Washington sells weapons to Taipei despite failing to recognise Taiwan as an independent state.
What is the Taiwan-China dispute about?
The rift goes back to China's civil war in 1927, which pitted forces aligned with the Communist Party of China against the Nationalist Kuomintang (KMT) army.
Eventually defeated by Mao Zedong's Communists, KMT chief Chiang Kai-shek fled to Taiwan, which was still under KMT control.
From there, Chiang continued to claim the entirety of China – just as the mainland claimed Taiwan.