The construction crew peered into two inky shafts bored into a rocky cliff face, bracing against the bitter cold and the impending blast.

Harpal Singh gave the go-ahead. The mountain was rocked by a thunderous explosion and, moments later, patriotic cheers. India was one step closer to completing a top strategic priority: a series of new tunnels and roads leading to the increasingly militarized border with China.

The tunnel will “safeguard the territorial integrity of our motherland,” said Singh, an engineer overseeing about 1,700 men racing to finish one stretch of the $600 million upgrades. Down the twisting one-lane road from their worksite were parked construction machines, heavy trucks hauling winter supplies for the army, and armored vehicles under a camouflage-patterned tarp, all preparing to make an arduous drive to the border that will become substantially shorter once the construction is finished.

“We understand the importance of this project,” Singh said. “It’s the vital supply line to the border with China.”

High in this corner of the Himalayas, an expanse of snowy peaks and glacier-fed rivers claimed by both China and India, a tense standoff between the two armies is spurring a flurry of infrastructure and military buildup that’s transforming one of the world’s remotest and most inhospitable regions.

On the Chinese side of the unmarked border, new helicopter pads, runways and railroads have been laid on the Tibetan plateau, according to satellite images and state media reports. On the Indian side, officials are rushing construction on the Zoji La tunnels, upgrading several strategic roads and unveiling new cellphone towers and landing strips. Both countries have deployed more military force to the border, with India diverting nearly 50,000 mountainous-warfare troops there, according to current and former Indian military officials. In recent months, both militaries have publicized combat-readiness drills to practice airlifting thousands of soldiers to the front lines at a moment’s notice.

Following 13 rounds of inconclusive negotiations between military commanders since June 2020, the standoff is now entering a second winter, an unprecedented development that is stretching logistics and budgets — especially for India. But the result, observers say, is a normalization of a hardened border, and an uneasy stalemate between two Asian powers that could last for years.

Retired Lt. Gen. Deependra Hooda, who served until 2016 as head of the Indian army’s Northern Command, said India last year assigned, for the first time, an offensively oriented mountain warfare division to the China border.

“The thinking was always we could handle China politically, diplomatically, but that feeling changed after 2020,” said Hooda, who directs the Council for Strategic and Defense Research think tank in New Delhi.

The deployed troops require “huge infrastructure to support them, huge reserves to replace them,” he added. “But even if there is a diplomatic process, the fact is that suspicions are going to remain. There’s no way of returning to the status quo.”

Over the years, the question of where India ends and China begins has been the subject of negotiations by various parties — including the British Empire and the Qing Dynasty, and an independent India under Prime Minister Jawaharlal Nehru and Communist China under Mao Zedong. The roughly 2,100-mile border does not cleave through significant natural resources or population centers, but the dispute over where exactly it lies has led to a bloody war and several skirmishes.

The current impasse began in May 2020, when Chinese patrols objected to Indian construction on a strategically placed road in Ladakh, inside territory that China claimed. The faceoff culminated in a brawl that killed scores of soldiers that June.

Infrastructure construction also sparked earlier flare-ups. In 1957, China built a road linking the two vast and restive regions in its west — Tibet and Xinjiang — that crisscrossed an expanse of salt flats that India claims to be part of Ladakh. Tensions over the road simmered and contributed to the Sino-Indian War of 1962, in which China attacked India and both sides saw thousands of men die in freezing conditions.

In 2017, Chinese workers sought to build roads near India’s Sikkim state before they were stopped by Indian troops, creating a diplomatic crisis.