China and India have stumbled once again into a bloody clash over some of the most inhospitable terrain on Earth.

A deadly brawl last month killed 20 Indian border troops and an unknown number of Chinese soldiers, punctuating a decades-old border dispute that has become one of the world’s most intractable geopolitical conflicts. It has inflamed tensions at a time when the world is consumed by the coronavirus pandemic, and it has scuttled recent efforts by the two Asian powers to set aside their historical differences.

In the weeks since, the two sides have tried to walk back from the brink, with military commanders and senior diplomats negotiating quietly to disengage. By late last week, satellite photographs indicated that Chinese troops had pulled out of one disputed area where a brawl sparked the latest tensions.

Even so, the broader dispute between the world’s two most populous nations, both armed with nuclear weapons, remains unresolved and dangerous. It involves a region called Ladakh, a sparsely populated area, high in the Himalayas, with close historical and cultural ties to Tibet. It was divided in the years after India gained independence from Britain in 1947 and the Communist Party established the People’s Republic of China two years later.

During its invasion of Tibet in 1950, Mao Zedong’s China seized the northern part of Ladakh, called Aksai Chin, and has held it ever since — in no small part because a crucial road connecting Tibet with another restive province, Xinjiang, runs through it. In 1962, the two countries went to war over the same terrain, but despite an overwhelming Chinese victory, the de facto frontier — known as the Line of Actual Control — remained roughly the same.

The clashes this spring and summer stemmed from India’s recent efforts to build up the road network on its side of the frontier, catching up — belatedly, critics say — to China’s buildup on its side. Last year, India completed an all-weather road connecting Leh, the capital of Ladakh, to its northernmost outpost at Daulat Beg Oldi. In the last two decades, India has constructed nearly 5,000 kilometers of roads, allowing it to move military forces more easily along the mountainous border region.

China appeared alarmed by that and by India’s decision last year to impose direct national rule over the Ladakh region.

“China is very sensitive to Indian activity in the western sector,” said M. Taylor Fravel, director of the Security Studies Program at the Massachusetts Institute of Technology, “and it goes back to the reasons why it decided to fight in 1962 — to defend that road that connected Xinjiang to Tibet.”

Galwan Valley is not the only hotspot along the frontier. By late April and early May, Indian troops began to observe a buildup of Chinese forces at two other spots along the Line of Actual Control: Pangong Lake and Hot Springs.

While no clashes occurred in Hot Springs, the Chinese brought up significant weaponry. About three kilometers away from the Line of Actual Control, companies of tanks and batteries of towed artillery appeared in existing Chinese positions north and east of Gogra.

The tensions this year first boiled over on the northern shore of Pangong Lake, a glacial lake split by the de facto border.

In early May, troops from both countries brawled in disputed territory there. There were a number of injuries, some serious, though no deaths. That fight put both sides on edge, contributing to the deadly clash in the Galwan Valley a little more than a month later. Years ago, the two countries agreed that their troops should not shoot at each other during border standoffs. But China seems to be testing the limits. In the June fighting, Indian commanders said that Chinese troops used iron clubs bristling with spikes.

China’s actions in the Himalayas have mirrored similar efforts to assert or reinforce its territorial claims, especially in the South China Sea. Chinese warships have this year menaced fishing and research vessels from Vietnam, Malaysia and Indonesia. In recent weeks, China is reported to have expanded its territorial claims in Bhutan, which has a close defense relationship with India.

Some analysts have argued that China is acting while the world is distracted by the coronavirus pandemic; others say China needs to distract its own population with nationalist propaganda about defending Chinese sovereignty. In any case, the tensions are unlikely to diminish.