Construction crews work at the Sela Tunnel in Arunachal Pradesh in 2021.
Hong Kong CNN  — 

A tunnel constructed high in the mountains of northeastern India has become the latest flashpoint in a simmering border dispute between New Delhi and Beijing.

The Sela Tunnel, inaugurated by Indian Prime Minister Narendra Modi earlier this month, has been hailed in India as a feat of engineering – blasted through the Himalayas at an elevation of some 13,000 feet (3,900 meters) – and a boon for the military, enabling faster, “all-weather” access to a tense de facto border with China.

That’s caught the attention of Beijing, whose long-running dispute with New Delhi over their contested 2,100-mile (3,379-kilometer) border has seen the two nuclear-armed powers clash in recent years.

That includes in 2020 when hand-to-hand fighting between the two sides resulted in the deaths of at least 20 Indian and four Chinese soldiers in Aksai Chin-Ladakh in the western stretches of the border.

And, decades ago, the dispute led to war.

China also claims the Indian state of Arunachal Pradesh, where the tunnel was constructed, as its own, even as the area has long functioned as Indian territory.

Chinese officials in recent days have slammed the tunnel project and Modi’s visit to the state, accusing New Delhi of taking steps to undermine peace along the border.

“We require the Indian side to cease any action that may complicate the boundary question … the Chinese military remains highly vigilant and will resolutely defend national sovereignty and territorial integrity,” a Defense Ministry spokesperson said last week, using the Chinese name “Zangnan” or South Tibet to refer to Arunachal Pradesh.

India hit back on Tuesday, slamming Beijing’s “absurd claims” and saying the area “is and will always be an integral and inalienable part of India.”

The US State Department also weighed in during a news conference Wednesday backing India’s sovereignty over Arunachal Pradesh and voicing strong opposition to “any unilateral attempts to advance territorial claims by incursions or encroachments” across the line of actual control (LAC), or de facto border.

Beijing hit back at this too, accusing Washington of sparing “no efforts to provoke and take advantage of other countries’ conflicts to serve its selfish geopolitical interests.”

The spat – which underscores the deep tensions undergirding the relationship between Asia’s two largest countries – comes as India is weeks away from national elections expected to deliver a resounding endorsement of Modi’s Hindu nationalist platform.

Rising nationalism under Modi has come alongside a similar phenomenon in China, where leader Xi Jinping has overseen an assertive foreign policy – though both sides have appeared to take steps to cool border tensions following the deadly 2020 clash.

Modi on Friday also traveled to Bhutan for a trip “aimed at further cementing the India-Bhutan partnership,” he wrote on social media platform X. The remote Himalayan country bordering Arunachal Pradesh too has a contested border with China, and New Delhi has been wary of a potential settlement between leaders there and in China over that dispute.

Indian soldiers stand guard along a highway in Ladakh in 2022.

Entrenched dispute

During his visit to Arunachal Pradesh earlier this month, Modi praised the Sela Tunnel as an “engineering marvel,” while touting a host of other development projects, including those related to border infrastructure.

Those projects are part of the intensified push by his government to develop Indian territory along the contested border. That push – which Beijing has signaled it is closely watching – reverses longstanding thinking from the Indian government that not developing the harsh terrain would have a deterrent effect on invasion or encroachment by China.

Of 118 projects launched by India’s Border Roads Organization last year to build infrastructure like roads, bridges and airfields, more than half were in Arunachal Pradesh and Ladakh, along the disputed border with China.

Beijing has accused India of “complicating the boundary question and disrupting the situation in the border areas between the two countries” with such development.

But observers say India is addressing an imbalance after decades of Chinese road and infrastructure building gave it a significant advantage over New Delhi in deploying troops to border areas, where it also constructed hundreds of “xiaokang,” or villages, which Beijing denies are meant to enforce its land claims.

“Now that India has recognized the benefits of border infrastructure, it is accelerating its building efforts and playing catch-up to China. But its efforts are also likely to raise tensions with China and encourage Beijing to also redouble its own building efforts,” said Byron Chong, a research associate at the Lee Kuan Yew School of Public Policy’s Center on Asia and Globalization in Singapore.

The Sela Tunnel, which runs from the state of Assam to Arunachal Pradesh’s Tawang, has likely caught special attention from Beijing given the sensitivity of the area, which lies along the line of actual control. New Delhi has said the project will “boost the preparedness of the Armed Forces.”

Tawang was the site of a non-lethal scuffle between the two sides in late 2022, according to Indian authorities. Then, New Delhi accused China’s People’s Liberation Army troops of trying to “unilaterally” change the status quo by attempting to cross the LAC.

But Tawang may also be especially important to Beijing given its significance within Tibetan Buddhism and China’s concerns surrounding the succession of spiritual leader the Dalai Lama, according to Manoj Kewalramani, who heads Indo-Pacific studies at the Takshashila Institution research center in Bangalore.

The Dalai Lama, 88, has lived in exile in India since a failed 1959 uprising against Chinese rule of Tibet, where Beijing has cracked down on religious practice and sought to exert its control over Tibetan Buddhism.

“A lot of this pressure (on Arunachal Pradesh) right now is a product of anticipation of when that event takes place … (in terms of) how China wants India to respond to it and what could be the potential fallout,” he said.

But even as the two sides build-up around the border – and China continues a push to entrench its territorial claims in its maps and official language, they have made diplomatic efforts to quell tensions following the deadly 2020 clash.

During a meeting on the sidelines of the BRICS summit in South Africa last August, Modi and Xi agreed to “intensify efforts” to deescalate tensions.

The Indian and Chinese militaries have also continued to hold border talks, the most recent of which took place last month, when they reiterated a commitment to “maintain peace and tranquility.”