WASHINGTON (BLOOMBERG) - It didn't take long for Donald Trump to throw out the long-established diplomatic rulebook on Taiwan: A month before taking office in January 2017, he accepted a telephone call from President Tsai Ing-wen.

That 10-minute conversation amounted to the first time a president-elect had spoken to a Taiwanese head of state since the US cut ties with Taipei in favour of Beijing in 1979, and it infuriated President Xi Jinping's administration.

Trump threw fuel on the fire a few days later by questioning whether the US needed to abide by its "one-China" policy.

While Trump soon backpedalled, displaying an inconsistency that has become a hallmark of his presidency, it showed China that relations with the US were changing quickly.

And Sunday's visit by Health and Human Services Secretary Alex Azar, the highest-level trip since the US recognised China 41 years ago, marks yet another step toward America treating Taiwan like any other country in the world - an outcome that Beijing has threatened could lead to war.

So far China's response to the trip has been fairly subdued, suggesting that leaders in Beijing may be keeping their options open ahead of the election, particularly as Trump struggles in the polls.

It comes at a time when the world's two biggest economies are struggling to boost growth in the wake of the coronavirus pandemic and handle social unrest.

While a certain amount of nationalism can help both leaders, anything that could trigger a war would be devastating.

Even as the Communist Party's nationalist tabloid Global Times published a scathing editorial on Wednesday night warning China could play the "military card," the official mouthpiece People's Daily appeared much more restrained.

On Thursday, the paper carried a statement on a bottom corner of the 11th page from Ma Xiaoguang, spokesman from China's Taiwan Affairs Office.

He urged the US to adhere to Beijing's one-China principle and the three joint communiques underpinning ties between the countries, while also hitting out at Tsai's ruling party in Taiwan.

"Taiwan's Democratic Progressive Party is willing to become a chess piece and collude with the US in a bid to seek political gains," Ma was quoted as saying.

"This is a very dangerous move and will not succeed."

Foreign Ministry spokesman Wang Wenbin repeated the government's opposition to official US-Taiwan interactions, while saying China "will take strong countermeasures in response to US behaviour."

China views separately ruled Taiwan as part of its territory and regularly threatens to bring it under its control by force if necessary.

Taiwan's government rejects Beijing's claim, asserting the island is already de facto independent state.

'SALAMI SLICING'

China has long referred to Taiwan as the most sensitive issue in relations with the US, and warned the Trump administration against breaching its "red line."

But just where that line sits is unclear.

"It's not easy for an individual scholar to answer questions about the bottom line," said Liu Guoshen, director of Xiamen University's Taiwan Research Institute.

"In principle, the official response is the bottom line, but how to grasp specific indicators is an art. It needs comprehensive evaluation and decision makers will make the judgment."

Now American politicians are testing China's bottom line using a page from Beijing's playbook.

Neighbouring countries have accused China of "salami slicing" - taking small steps over time - to strengthen territorial claims in the South China Sea, India and elsewhere.

And now China is accusing the US of using the same tactics with Taiwan: In June, Defence Ministry spokesman Wu Qian used the term while saying the People's Liberation Army is on high alert, and the Global Times used it again this week to warn Taiwan it could face an "unbearable disaster" if it serves as an American "pawn."

When Tsai was first elected president in 2016, she expressed a willingness to talk to Beijing but refused to accept the notion that Taiwan is part of China.

China responding by cutting off direct contact between the governments and resuming its efforts to persuade Taiwan's few remaining diplomatic allies to switch recognition to Beijing after an eight-year diplomatic truce under Tsai's China-friendly predecessor Ma Ying-jeou.

After Sao Tome and Principe and Panama switched allegiance in 2017, the US began taking a tougher line against Chinese efforts to isolate Taiwan.

In January 2018, the US House passed a pair of bills seeking greater diplomatic support for Taiwan: One encouraged visits between the US and Taiwan "at all levels," including "cabinet-level national security officials."

The other urged the US State Department to develop a strategy to help Taiwan regain its observer status in the World Health Organisation.

At the same time, Beijing stepped up its policing of international companies such as Inditex SA-owned Zara and Delta Air Lines and Marriott International, demanding they clearly label Taiwan and Tibet as part of China on their websites and any labelling.

China has also increased its patrols of military aircraft and vessels around Taiwan over the past few years, regularly encroaching into Taiwan's air defence identification zone and occasionally crossing the middle line of the strait.

The US has responded in kind, sending its warships through the strait in what it deems freedom of navigation operations.

The Trump administration has also made moves to bolster Taiwan's ability to defend itself, approving the sales of billions in advanced weaponry over the past two years, including 66 F-16s - the first sale of fighter jets to Taiwan since the early 1990s.

Taiwan came up in a phone call on Thursday night between Chinese Defence Minister Wei Fenghe and US Defence Secretary Mark Esper.

"Both leaders agreed on the importance of maintaining open channels of communication and developing the systems necessary for crisis communications and risk reduction," the Pentagon said in a readout of the call.

The US moves have largely been in response to China's pressure on Tsai's government, according to Kharis Templeman, a visiting fellow at the Hoover Institution.

"Beijing likes to play this game with Taiwan where they make a series of small changes to ratchet up pressure," he said.

"Well, now the US has shown repeatedly that it can respond in kind. If Beijing doesn't like cabinet-level visits, maybe they should stop sending military aircraft into Taiwan's Adiz."