With both sides lodging complaints, yet Japan insisting its long-held stance has not changed, AFP examines Takaichi's remarks, their significance and the resulting furore.

What did Takaichi say?

Before taking power last month, Takaichi, an acolyte of ex-premier Shinzo Abe, was a vocal critic of China and its military build-up in the Asia-Pacific.

She has been a regular at a shrine that honours Japan's war dead -- including those of guilty of crimes in World War II -- that is seen by Asian nations as a symbol of Japan's militaristic past.

Japan's first woman prime minister has visited Taiwan in the past and met Taipei's representative at a recent APEC summit. She also held talks with Chinese President Xi Jinping at the same event.

Her comments on November 7 were widely interpreted as implying that an attack on Taiwan, which is just 100 kilometres (62 miles) from the nearest Japanese island, could warrant Tokyo's military support.

If a Taiwan emergency entails "battleships and the use of force, then that could constitute a situation threatening the survival (of Japan), any way you slice it," Takaichi told parliament.

"The so-called Taiwan contingency has become so serious that we have to anticipate the worst-case scenario," said Takaichi.

Japan's self-imposed rules say that it can only act militarily under certain conditions, including an existential threat.

Wang Hung-jen, a political analyst at National Cheng Kung University in Taiwan, said the comments "sent a very strong message to China: Japan is no longer just standing by and watching".

"The probability of Japan intervening to deter China and prevent any military action by China around the Taiwan Strait has increased significantly," he told AFP.

How did China react?

Beijing insists Taiwan -- which was occupied for decades by Japan until 1945 -- is part of its territory and has not ruled out the use of force to seize control.

In a now-removed post on X, the Chinese consul general in Osaka Xue Jian threatened to "cut off that dirty neck", apparently referring to Takaichi.

Tokyo lodged a protest and Takaichi's ruling party passed a resolution calling for the envoy to be declared persona non grata.

On Wednesday Yuyuan Tantian, a Chinese outlet linked to state broadcaster CCTV, said Takaichi "will likely have to pay the price" for her comments.

Beijing's foreign ministry on Thursday called on Japan to "retract the unjustified remarks", warning otherwise that the "consequences... must be borne by the Japanese side".

A day later, China said it had summoned the Japanese ambassador to make "serious demarches" about the "erroneous remarks".

"If anyone dares to interfere with China's unification cause in any form, China will surely strike back hard," the foreign ministry said.

In response, Japan said that its position on Taiwan was "unchanged" and that it continues to urge "peace and stability" in the Taiwan Strait.

Tokyo on Friday also summoned the Chinese ambassador to protest Xue's "extremely inappropriate" threat.

Why does it matter?

Tokyo normalised diplomatic ties with Beijing in 1972 but relations have long been dogged by historical issues. Japan also has a warm relationship with Taipei.

Previous sitting Japanese prime ministers have avoided commenting directly on Taiwan's defence, choosing instead to maintain "strategic ambiguity".

Similarly, the United States has long been deliberately ambiguous on whether it would deploy its military to defend Taiwan, although former president Joe Biden repeatedly suggested he would order the US military to intervene if China moved on the island.

Yee Kuang Heng, a professor at the University of Tokyo, told AFP that Takaichi's comments may have been her "own personal inclination" to signal a stronger stance.

But although "clearer communication can enhance deterrence" he said, he also warned that "a delicate balance has to be struck with the flip side of the coin, which is keeping the other side guessing."

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