NEW DELHI — At least 20 Indian soldiers were killed in a fierce clash with Chinese troops on Monday night high in the Himalayas. It was the deadliest conflict between the nuclear-armed neighbors in more than 50 years.

Each country accused the other of sparking the clash in a remote river valley where the precise border between the two nations is disputed. China has reported “casualties” in the clash but has not disclosed the number of its troops wounded or killed. The foreign ministers of the two countries spoke Wednesday to try to reduce friction, but the situation remains tense and uncertain.

Here is what you need to know about the conflict:

What is the source of the current tension?

India and China share a 2,200-mile border, much of it crossing remote, mountainous regions. In many areas, the frontier remains a matter of interpretation, with both countries making competing territorial claims. For decades, the two countries have managed peacefully to settle flare-ups on the disputed border — known as the Line of Actual Control.

Since May, however, Chinese troops have camped in territory claimed by India in Ladakh, recent reports suggest. One of those points is in the Galwan River valley that analysts say overlooks a road that is crucial for India. Last month, Indian and Chinese troops also engaged in physical skirmishes, both in Ladakh and hundreds of miles away in Sikkim, leaving dozens injured.

Monday’s clash took place in Ladakh’s Galwan valley. Neither country has been forthcoming with details of what transpired. No weapons were fired, said an Indian military official in Delhi who spoke on the condition of anonymity because he was not authorized to discuss the matter. The deadly brawl involved clubs, rods and stones.

Prime Minister Narendra Modi said June 17 that India was capable of giving a befitting reply to China after deadly clashes with Chinese troops. (Reuters)

Have India and China fought previously?

Relations between India and China have been marked by mutual distrust. In 1962, China attacked India, and the two countries fought a month-long war, which India lost. Nearly 4,000 Indian troops were killed. China viewed India’s support for the Dalai Lama and its offer of asylum to Tibetan refugees as a provocation.

Five years later, in 1967, the two countries clashed again when India began to put up wire fencing on the border. Dozens of troops from both countries were killed. The last time there were combat casualties was in 1975, when Chinese troops ambushed four Indian soldiers in the state of Arunachal Pradesh in India’s northeast.

Since then, the border has remained peaceful, although not without friction. In 2017, China began extending a road on the border with India and Bhutan — a move that was blocked by Indian soldiers. The two-month standoff at Doklam ended when the two sides agreed to withdraw forces after diplomatic talks. Neither side admitted to backing down.

How do China and India compare?

India and China are the world’s most populous countries and have two of the world’s largest armies. With more than 1.3 billion people, India is set to overtake China as the world’s most populous country during this decade.

But as China has emerged as a global power, India’s economic growth has not kept pace. India’s economic output was $2.7 trillion in 2018, a value that China exceeded more than a decade ago.

China spends a smaller percentage of its GDP on the military than India. However, in absolute numbers, China’s military budget is more than three times that of India’s. China’s active troop strength is more than 2 million, and India has nearly 1.4 million active military personnel.

What are other sources of friction between India and China?

India has watched China’s rising economic strength and geopolitical influence with a wary eye. India dislikes China’s efforts to draw closer to India’s neighbors, including Pakistan, Sri Lanka and Nepal.

India has also stayed away from China’s ambitious Belt and Road Initiative, which has invested in infrastructure projects in the region. India objects to China’s effort to build an economic corridor in Pakistani-administered Kashmir. The territory is claimed by both Pakistan and India.

China, meanwhile, is displeased with India’s growing closeness with the United States, as well as its ongoing support for the Dalai Lama. The recent tensions occurred despite a charm initiative by Indian Prime Minister Narendra Modi, who has made five visits to China since his election in 2014, the most by any Indian leader.

Where does the United States stand?

The United States is “closely monitoring the situation between Indian and Chinese forces along the Line of Actual Control,” a State Department official said Tuesday, speaking on the condition of anonymity under department policy. “Both India and China have expressed a desire to de-escalate, and we support a peaceful resolution of the current situation.”

In May, President Trump offered to “mediate or arbitrate” between the two countries, an offer rejected by both. He described the situation as a “raging border dispute.”

Trump has dueled with China, holding it responsible for the spread of the coronavirus pandemic. He has hailed Modi as a “great gentleman.”

What happens next between China and India?

Monday’s clash presents a new challenge, but neither India nor China appears willing to risk a broader conflict. India is struggling to contain the coronavirus pandemic and an economic crisis, and China is grappling with sharply slowing economic growth and a spiraling rivalry with a far more threatening geopolitical opponent, the United States.

Correction: An earlier version of this story included a figure for India’s gross domestic product adjusted for purchasing power parity. The actual figure is $2.7 trillion.

Shams Irfan in Srinagar and Gerry Shih in Seoul contributed to this report.