When Donald Trump took a call from Taiwan’s Tsai Ing-Wen, he became the first president or president-elect in 40 years to speak to the island’s leader.

The call has plunged the superpowers into a diplomatic incident over a key foundation of US-China relations: American support for the "One China" policy.

The island of Taiwan is democratically-governed, and consistently ranks highly in terms of healthcare, press freedom and public education, but is considered by China to be a breakaway province, not an independent state free to talk to the wider international community.

Within 24 hours of the call, Beijing criticised the incoming president’s freewheeling diplomatic style and reminded the US of its historic commitment to One China.

Mr Trump doubled down in a Fox News interview on Sunday, saying the US’ future position on Taiwan could be tied to trade deals. The Chinese government said it was “seriously concerned” by the policy shift.

What does 'One China' mean?

Since the Chinese Civil War of 1949, Taiwan has been the home of the government of The Republic of China (ROC), which retreated to the island after defeat on the mainland by Mao Zedong’s Communist Party.

The victorious People’s Republic of China (PRC), which took a seat on the UN Security Council in 1971, also laid claim to Taiwan, forcing the international community to accept a divided China, and commit to diplomatic relations with one side or the other.

During the Cold War, Western states retained diplomatic ties to the ROC, which saw itself as the last stronghold of a country overrun by Communist rebels, while the Soviet Union and its allies supported and traded with the Communist PRC.

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The two sides struck a formal agreement in 1992, under which there is only one China, but agreed to disagree on which government was legitimate.

The PRC’s constitution says it is "the lofty duty of the entire Chinese people, including our compatriots in Taiwan, to accomplish the great task of reunifying the motherland", and insists that any country seeking diplomatic relations with them must agree to their version of One China.

This includes severing all diplomatic contact with Taiwan. As a result, Taiwan is recognised officially by just 21 countries, as well as the Vatican. The most significant of those, Paraguay, is the world's 100th largest economy, according to the World Bank.

What has been the US stance toward 'One China'?

In 1972, President Richard Nixon signed the first of three ‘joint communiques’ laying out the US’ official stance on Taiwan. It read: “Taiwan is a part of China. The United States does not challenge that position.”

But the US still dealt with the ROC government in Taipei until 1979, when President Jimmy Carter switched the diplomatic focus of the US from Taipei to Beijing.

Mr Carter’s administration maintained relations with Taiwan but stopped short of officially recognising the ROC as the government of an independent state. Mr Carter was the last US President to speak to a Taiwanese leader.

In 1989, President George W Bush told a state dinner in Beijing that "based on the bedrock principle that there is but One China, we have found ways to address Taiwan constructively, without rancor".

Even while President Obama angered the Chinese authorities by repeatedly meeting with the Dalai Lama, spiritual leader of the contested province of Tibet, a White House spokesman told the Taipei Times in September that “we remain firmly committed to our One China policy” - a position which Mr Trump looks set to reverse.

What is Mr Trump hoping to get out of Taiwan?

The President-elect considers himself the arch-dealmaker, and could be hoping to use Taiwan to extract concessions from Beijing on other issues.

His campaigning rhetoric toward China has been hostile: in May, Mr Trump accused China of "raping our country" over the US' trade deficit, while he took to Twitter in typically antagonistic fashion after the call with Taipei:

In re-opening an old wound over Taiwan, Mr Trump is positioning himself at the negotiating table, and using One China as leverage.

Joanna Lei, head of Taipei's Chung Hua 21th Century Think Tank, told CNN: "being the lever isn't a good place for Taiwan."

Taipei might sleep easier believing the renewed focus on their predicament is a just a factor of Mr Trump's irreverent style, and his mandate to shake up the established order.

Xenia Wickett, head of the US and Americas Project at international relations think tank Chatham House, said "we shouldn't be surprised" at Mr Trump's behaviour over Taiwan.

She told The Independent: "He says 'I have no preconception, no ideology', so it should be no great surprise that he's said ‘why shouldn't I talk to this other country? I have nothing against them'.

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"Entering into this without preconceptions, without a sense that what's gone before should matter - that does give him a strength. And now the Chinese are giving him wiggle room, because they don't know how seriously he's taking this. This plays to his desire for unpredictability.”

Unpredictability may wrongfoot Mr Trump's opponents in the short-term, but Ms Wickett believes an unpredictable America could ultimately play into Chinese hands.

"Trump plays the short game," she said.

"The Chinese play the long game."

Could Mr Trump really negotiate on 'One China'?

The billlionaire businessman has already made it clear that no topic is off the table. But an editorial by the Global Times, a tabloid mouthpiece for the country's ruling Communist Party, likened any negotiation over the One China policy to putting a price on the US constitution.

It said “the One China policy is not for selling", called Mr Trump “naive", and advocated a “hard struggle to let him know that China and other world powers cannot be easily taken advantage of”.

Professor Rosemary Foot, of the University of Oxford, told The Independent the US had too much vested interest in China for Mr Trump to turn his words into action.

She said: "Many individual states in the US count China as a really important economic market.

"Also China knows that many of its neighbours do not want to see intensified rivalry between the US and China. If Trump raises the tension unduly, these same states will stop projecting the United States as the source of stability in the region, and move closer to China."

Ultimately, says Professor Foot, "China is not in the business of a trade on this issue. The recovery of Taiwan is a core interest."

A second editorial by Global Times on Monday warned that “the strength gap between China and the US for the moment is the narrowest in history”.