Chinese officials in Beijing are growing weary of early morning wake-up calls about Donald Trump. Three times this month the Chinese capital has risen to worrying news. First, the US president-elect received a phone call from the president of Taiwan. During the fallout over that protocol-busting conversation, Mr Trump took to Twitter to rebuke Beijing over its military activities in the South China Sea and alleged currency manipulation. Then, in an interview on Sunday, he suggested he may scrap the “One China” policy, the central pillar of Sino-US relations since 1972.

What is the One China policy?

In 1949, after losing a protracted civil war to Mao Zedong’s Communist party, Chiang Kai-shek’s Kuomintang, or Nationalist party, retreated to the island of Taiwan, about 180km off the mainland’s south-east coast. Ever since, the People’s Republic of China has insisted that Taiwan is simply a province of China. Any country wishing to establish diplomatic relations with Beijing must acknowledge there is only “One China” and sever all formal ties with Taiwan.

Officially, the “Republic of China” government in Taiwan still claims sovereignty over the mainland (and even Mongolia). But Beijing and Taipei have long adhered to the “1992 consensus” in which both sides agreed there was only China, while disagreeing on which republic was its rightful representative.

For the past 20 years, however, Taiwan has promoted itself as a free, democratic and in effect independent country. That is especially true when the pro-independence Democratic Progressive party is in power. The DPP’s Tsai Ing-wen defeated her KMT opponent by a landslide in January’s presidential elections.

Why did Trump’s December 2 phone call with Tsai set off such a firestorm?

Until this month there had been no official communication between a US president or president-elect and a Taiwanese leader since 1979, when Beijing and Washington formally established diplomatic relations.

That breakthrough was set in motion by President Richard Nixon and his secretary of state, Henry Kissinger, who began courting China in the early 1970s as a way of isolating the Soviet Union, which had its own falling out with Mao in the early 1960s. Both Nixon and Mao recognised that “the enemy of my enemy is my friend”.

In 1972, China and the US signed the first of three communiqués that have guided the relationship ever since. In the first, also known as the Shanghai communiqué, Beijing said “the Taiwan question is the crucial question obstructing the normalising of [Sino-US] relations”.

For its part, the US acknowledged only that “all Chinese on either side of the Taiwan Strait maintain there is but one China and that Taiwan is a part of China”. But Congress also passed legislation requiring the US to sell Taiwan military equipment necessary for its defence. US arms sales to Taiwan remain a source of friction with Beijing.

Why shouldn’t the US recognise free and democratic Taiwan as independent?

For the Chinese Communist party, recovering Taiwan is not just unfinished business from an unfinished civil war. It runs much deeper in the Chinese psyche than that.

Chinese history has for centuries veered between times of luan, or chaos, when the country was fragmented, and glorious periods when it was united and prosperous under great dynasties that enjoyed “the mandate of heaven”. The “great rejuvenation of the Chinese nation” promised by President Xi Jinping will be incomplete so long as Taiwan remains outside the fold.

It is also an important part of the Communist party’s claim to political legitimacy, alongside economic prosperity. According to Taiwan media reports, Mr Xi recently told a visiting KMT delegation that his party would be “overthrown” if it allowed Taiwan to become independent.