Since 1979, the US has agreed to recognise the “One China” policy.
It means the US acknowledges China’s position that there’s only one Chinese government.
So what does that mean for Taiwan?
Under the policy, the US has formal ties with China, not Taiwan.
But the US still supports the island and has promised to help it defend itself, including supplying arms.
The "One China" policy is a key cornerstone of Sino-US relations. But it’s different to the One China principle, which is China’s insistence that it will one day fully take back Taiwan.
********************* Who recognises Taiwan?
China regards Taiwan as a breakaway province which it has vowed to retake, by force if necessary.
But Taiwan's leaders say it is clearly much more than a province, arguing that it is a sovereign state.
It has its own constitution, democratically-elected leaders, and about 300,000 active troops in its armed forces.Chiang Kai-shek's Republic of China(ROC) government, which fled the mainland to Taiwan in 1949, at first claimed to represent the whole of China, which it intended to re-occupy.
It held China's seat on the United Nations Security Council and was recognised by many Western nations as the only Chinese government.
But in 1971, the UN switched diplomatic recognition to Beijing and the ROC government was forced out.
Since then the number of countries that recognise the ROC government diplomatically has fallen drastically to about 15.
Given the huge divide between these two positions, most other countries seem happy to accept the current ambiguity, whereby Taiwan has virtually all of the characteristics of an independent state, even if its legal status remains unclear.
********************** Taiwan among 'freest societies in the world' - Pelosi
Pelosi has been speaking in the Taiwanese parliament - she said she wanted to increase parliamentary exchanges between the US and Taiwan.
She also described Taiwan as "one of the freest societies in the world".
She also said a US bill on computer chips - which will see big US investment in domestic semiconductor manufacturing and science research - was a good opportunity for more cooperation with Taiwan.
Pelosi was received by vice president of the parliament, Tsai Chi-Chang.
China's ruling Communist Party has threatened to use force if Taiwan ever formally declares independence.
************************ Taiwan a rare issue with bipartisan US support
Nancy Pelosi’s visit is garnering a fair bit of bipartisan support from Republican opponents who’d usually be very unlikely to count themselves among her fanbase.
With former senior officials from the Trump administration Mike Pompeo and John Bolton tweeting their approval, it’s a reminder of how – regardless of the Biden administration’s attempts to dissuade her from travelling – China’s increasing threats over Taiwan are only serving to build a rare space for consensus in a political landscape normally riven by partisan division.
This week, the Senate Foreign Relations Committee is due to discuss a new piece of proposed legislation, the Taiwan Policy Act of 2022, crafted by both Democrat and Republican senators which would boost US military assistance to the island and designate it as a “major non-Nato ally”.
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'We're supporting trip' says Republican senator as Pelosi attracts bipartisan support
Republican Senator Dan Sullivan has become the latest of Nancy Pelosi's politica l opponents to support her trip to Taiwan.
"You do not want the Chinese Communist Party dictating to senior American leaders where they can and cannot travel," he told a news conference.
"My view - and I think the view of a number of senators - is that we should close ranks now and not let the Chinese Communist Party try to drive wedges between American elected officials or the White House on whether or not she should be there.
"She's there, we're supporting the trip and we're closing ranks behind her."
Diplomatically, the US currently sticks to the "One-China" policy,
which recognises only one Chinese government - in Beijing -
and has formal ties with China rather than Taiwan.
But in May, US president Joe Biden appeared to harden Washington's position.
Asked whether the US would defend Taiwan militarily, Mr Biden replied: "Yes."