For centuries, the glaciers of the Western Himalayas have fed the Indus River, which flows down the mountains through India and into Pakistan, where it runs the length of the country to the Arabian Sea. In both countries, the river is a crucial source of water for livestock, irrigation, drinking—essential to life and livelihood for millions of people.

But as climate change causes global temperatures to rise, the glaciers that feed the Indus are receding. A series of scientific reports indicates that in the coming decades, the river’s water levels could drop by as much as 40 percent. Already, some Indian policymakersare raising the idea of damming that water off for their own country. That could save the lives of millions of Indians, while threatening millions of Pakistanis. Pakistan lacks the economic, political, or conventional military leverage to retaliate against India if that happens; it matches its neighbor only in nuclear weapons.

National security agencies around the world, including the Pentagon and the CIA, are watching the situation closely, nervous that climate change could one day ignite a nuclear face-off between these two volatile neighbors.

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