The simmering China–India border faceoff in the Himalayas turned suddenly deadly late on Monday, with 20 Indian soldiers reportedly killed in high-altitude clashes with China’s People’s Liberation Army (PLA) forces, according to Indian officials. Chinese casualty figures have yet to be disclosed.

These are the first fatalities in engagements between the two Asian giants since 1975, when four Indian soldiers were killed in an ambush in Arunachal Pradesh. It is also the single deadliest day of the long-running border dispute since the Sino-Indian War of 1962, during which more than 2,000 military personnel were killed and India was forced to cede territory in Aksai Chin equivalent to the size of Switzerland.

The rising tensions this month were accompanied by repeated insistences by both Chinese and Indian officials that the situation was under control and that de-escalation was imminent. Indeed, when the first three casualties of the deadly clashes on Monday had been identified, Indian sources claimed that the incident had occurred “during the de-escalation process.” At this rate, the two sides could rapidly de-escalate themselves into an all-out war.

While Chinese Foreign Ministry spokesman Zhao Lijian accused Indian forces of precipitating the clash by crossing into Chinese-held territory and attacking Chinese troops, Indian analysts claimed that fighting happened on Indian territory. Supposedly, no shots were fired — soldiers instead battered each other with rocks, clubs, improvised weapons, and their bare hands.

Will this standoff grow into a much more lethal conflict? As Ameya Pratap Singh argued on The Diplomat back at the beginning of June, the situation now is substantially different from how it was in 1962. Most importantly, a series of five agreements issued in the intervening years, including most recently the Border Defense Cooperation Agreement in 2013, provides a basis and method for working out differences. In addition, China can ill afford to take its eyes off Hong Kong and Taiwan, while India’s main security concern remains its primary rival, Pakistan. Finally, the military arsenals of both countries have grown substantially since their previous war, and both now have large nuclear stockpiles, meaning any war would likely be far too costly to justify.

Of course, Singh made his argument while the border conflict still had zero fatalities. Even if we ignore Chinese casualties, 20 deaths is a substantial loss of life for a fight that supposedly hasn’t even begun yet. The two armies have been massing troops and materiel on the border as a show of resolve and a means of deterrence in what is right now the world’s most dangerous game of chicken. While this buildup may indeed discourage aggression, it makes the potential local, regional, and global fallout of open hostilities all the more dangerous. This week’s violence does not bode well.