India and China are locked in a war of words after a months-long period of calm in their relationship over the alleged harassment of a woman from India’s eastern state of Arunachal Pradesh by Chinese authorities.

China claims the territory of Arunachal Pradesh and calls the region southern Tibet, or Zangnan.

Here is what happened, what the tensions over Arunachal Pradesh are about and why the new tensions between the world’s two most populous nations are significant.

What happened?

Prema Wangjom Thongdok was travelling on her Indian passport from the United Kingdom, where she lives, to Japan on Friday. She stopped at Shanghai Pudong International Airport in China for what was supposed to be a three-hour layover.

Thongdok alleged that Shanghai airport authorities instead detained and harassed her for 18 hours because her passport mentioned her place of birth as Arunachal Pradesh.

She added that the Chinese authorities at the airport said her Indian passport was “invalid”, claiming that Arunachal Pradesh was actually part of China.

“I had actually gone past the immigration gate. It was e-gate, so I put my passport through and went to the security gates,” Thongdok told the India Today news magazine over videolink from Thailand’s capital, Bangkok.

“One of the officials came and started screaming, ‘India! India!’ with my name and singled me out,” she added.

When Thongdok asked the official why she was being taken out, she said the official told Thongdok that her Indian passport is not valid because she was born in Arunachal Pradesh. Thongdok added that the official said: “Arunachal is part of China. Your passport is invalid.”

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Thongdok said she responded: “Arunachal is part of India. I’ve never heard of it being part of China.”

She added that she was held up at the airport without being provided a timeline of how long the authorities would keep her.

Other Indian media outlets reported that Thongdok said she was pressured by airport authorities to buy a new ticket on China Eastern Airline and hinted that her passport would be returned only when she complied. This led to her losing money on missed flights and hotel reservations.

She added that with help from a friend in the UK, she eventually got in touch with the Indian consulate in Shanghai. Indian officials later escorted her onto a late-night flight out of the city.

The Indian Express newspaper reported that Thongdok had passed through Shanghai Pudong International Airport in October without any issues. It is unclear what was different this time.

Have similar incidents happened before?

In 2005, Beijing adopted a policy of issuing stapled visas to people from Arunachal Pradesh coming to China. Other Indian residents receive a stamped Chinese visa on their passports.

China’s argument: It cannot issue regular visas to people from Arunachal Pradesh when it considers them Chinese to start with.

But India has never accepted stapled Chinese visas on its passports. Doing so, it said, would be tantamount to accepting that people from Arunachal Pradesh are not Indian.

In 2013, two archers from Arunachal Pradesh were issued stapled visas  to travel to Wuxi in eastern China for the Youth World Archery Championship. India barred them from flying to China.

In July 2023, three martial arts athletes from Arunachal Pradesh were issued stapled visas for the World University Games, which took place in China’s Chengdu. The entire Indian wushu team as a result ended up not going to the games in Chengdu.

In September 2023, the same three wushu athletes from Arunachal Pradesh were unable to compete in the Hangzhou Asian Games because they were unable to download their accreditation cards, which were to serve as visas to visit China.

Indian Sports Minister Anurag Thakur cancelled his visit to Hangzhou for the inauguration of the games in protest.

What is the Arunachal Pradesh dispute about?

The India-China border dispute over Arunachal Pradesh is a legacy of the British Empire, Raj Verma, a nonresident scholar at the Sigur Center for Asian Studies at George Washington University, told Al Jazeera.

In March 1914, the McMahon Line was established during the Simla Convention in India, which involved negotiations between Tibet, China and the British Empire. The line is named after the chief British negotiator, Henry McMahon. While Chinese delegates attended the convention, they did not sign the main agreement and did not recognise the McMahon Line.

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McMahon nevertheless went ahead and decided on the line with Tibet. China has held that Tibet did not have the authority to unilaterally decide on the border with India, especially after China sent troops into Tibet and annexed it in 1951. Beijing has also turned to older maps, including from the British, that showed territory south of the McMahon Line as belonging to China.

Since gaining independence from Britain in 1947, India has treated the McMahon Line as the border with China.

Beijing has insisted that India and China need to negotiate and draw up their borders rather than relying on the British-era McMahon Line.

For decades, China claimed only a part of Arunachal Pradesh known as Tawang. But in recent years, its claim has extended to all of the northeastern Indian state.

Verma said tensions have intensified since Chinese President Xi Jinping came to power in 2012 and adopted a much more muscular approach to Beijing’s territorial demands.

“Our stance is steadfast and clear-cut when it comes to China’s sovereignty and territorial integrity. … We cannot lose one inch of territory passed down by our ancestors. Meanwhile, we want nothing from others,” Xi said in 2018.

China’s relations with many of its other neighbours have also soured over the past decade due to territorial disputes, including Beijing’s claims over all of the South China Sea.

“That [China’s posture] is the reason why it’s becoming more and more difficult for the dispute to be resolved, and it will be extremely difficult in the future for this issue to be resolved,” Verma said.

In India, Prime Minister Narendra Modi, in power since 2014, has also adopted a more assertive foreign policy, including on border disputes. He once said he would show “red eyes” – a metaphor in Hindi meaning to show firmness and strength – to China.

How has the Arunachal Pradesh dispute played out historically?

Arunachal Pradesh, India’s least populated state, has long been at the frontier of New Delhi-Beijing tensions.

In October 1962, present-day Arunachal Pradesh became one of the two theatres of the Indo-China War, and Beijing’s troops attacked Indian positions on both sides of the Namka Chu river.  Indian soldiers fled from Tawang although the Chinese People’s Liberation Army returned the territory to India after winning the war.

In October 1975, another skirmish broke out at the Tulung La pass, which connects Tibet and Arunachal Pradesh. Each side accused the other of attacking its soldiers. Four Indian soldiers were killed in what was the last deadly border clash between the neighbours until the 2020 crisis in Ladakh, in which at least 20 Indian soldiers and four Chinese service members were killed after hand-to-hand combat.

While the Arunachal Pradesh border has been largely peaceful for half a century, the competing Indian and Chinese claims over the region have played out in other ways.

In 2012, China objected to the Dalai Lama’s scheduled visit to Arunachal Pradesh. “We firmly oppose Dalai visiting the so-called ‘Arunachal Pradesh’,” said Jiang Yu, who was the spokeswoman for China’s Ministry of Foreign Affairs at the time. The Dalai Lama did not visit the state in 2012.

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In December 2022, minor border scuffles broke out in Tawang. Both sides blamed each other. While China accused Indian forces of obstructing a routine patrol, India alleged Chinese soldiers had encroached upon Indian territory and tried to “change the status quo”.

In January, China approved the construction of a dam in Medog County, Tibet, near the Indian border. India hit back, approving the Siang Upper Multipurpose Project in Arunachal Pradesh to offset the impacts of the Chinese dam and prevent flooding. However, the Indian reservoir project risks submerging dozens of Indigenous villages in India and Bangladesh.

Why is Arunachal Pradesh significant?

The state is strategically important to India because it links the country with Southeast Asia and is also a military district, Verma said. The state shares international borders with Myanmar to the southeast and is close to Bhutan to the west. Myanmar, in turn, connects India to Thailand, Laos and beyond via the India-Myanmar-Thailand Trilateral Highway project.

Verma said the state is also of religious significance to China. In 1683, the sixth Dalai Lama, Tsangyang Gyatso, was born near Tawang.

China insists that Arunachal Pradesh is an extension of Tibet, itself a source of tension between the neighbours: India’s decision in 1959 to give the current Dalai Lama sanctuary after he fled Lhasa during a Chinese crackdown has long been viewed by Beijing as a provocation.

In September 2024, India named a previously unnamed mountain peak in Arunachal Pradesh after the sixth Dalai Lama.

China voiced dismay about this decision. “Let me say more broadly that the area of Zangnan is Chinese territory, and it’s illegal and null and void for India to set up the so-called Arunachal Pradesh in Chinese territory. This has been China’s consistent position,” said Lin Jian, the Chinese Foreign Ministry spokesperson at the time.

“Beijing would be extremely reluctant to give up Arunachal Pradesh and the Tawang issue,” Verma said.

How have authorities in China and India reacted?

In response to a reporter’s question about Thongdok’s experience at the Shanghai airport, Chinese Foreign Ministry spokesperson Mao Ning said on Tuesday: “Zangnan is China’s territory. The Chinese side has never recognised the so-called Arunachal Pradesh, illegally set up by India.”

She insisted that Chinese officials had dealt with Thongdok “in accordance with laws and regulations”.

“The law enforcement was impartial and nonabusive. The lawful rights and interests of the person concerned were fully protected. No compulsory measures were taken on her, and there was no so-called detaining or harassing. The airline provided her with resting facilities and meals,” Mao said.

In response to his Chinese counterpart’s statements, Randhir Jaiswal, spokesperson for the Indian Ministry of External Affairs, said on Tuesday: “Arunachal Pradesh is an integral and inalienable part of India, and this is a self-evident fact. No amount of denial by the Chinese side is going to change this indisputable reality.”

Jaiswal added: “The issue of the detention has been taken up strongly with the Chinese side. Chinese authorities have still not been able to explain their actions, which are in violation of several conventions governing international air travel. The actions by the Chinese authorities also violate their own regulations that allow visa free transit up to 24 hours for nationals of all countries.”

What does this mean for India-China relations?

Verma said the latest dispute is “a minor blip in the bilateral relationship” between India and China. “It should not be an issue when it comes to improving India-China relations.”

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The relationship between New Delhi and Beijing, especially frigid from 2020 to 2024, has thawed over the past year. Both have been battered by heavy tariffs imposed by United States President Donald Trump, which analysts said have forced them to cooperate.

In 2020, India-China relations hit a low point after the Galwan Valley clash in Ladakh. This marked the first deadly border incident in 40 years and resulted in India banning 200 Chinese apps, including TikTok. India’s foreign minister called it the “most difficult phase” in decades.

Since late 2024, both nations have sought to reset ties. Modi and Xi met in Kazan, Russia, during a troop disengagement along the Line of Actual Control.

In August, Chinese Foreign Minister Wang Yi visited India, leading Modi to praise their “respect for each other’s interests” and “steady progress” in relations. Later that month, Modi made his first visit to China in more than seven years and met Xi for talks.

Still, Verma pointed out, despite the thaw in ties, “the overall strategy of both countries towards each other has not changed.”

“Both countries view each other as rivals, and the competition and rivalry between the two countries will continue,” he said.