Pakistan, June 18 -- Somewhere in the upper reaches of the Chenab, beyond the snowfields of Lahaul-Spiti, engineers are drilling an 8.7-kilometre tunnel through a Himalayan mountain. When completed, it will silently redirect water that has flowed toward Pakistan for millions of years, channelling it instead into India's Beas River system - away from the fields of Pakistani Punjab, away from the farmers who have depended on it for generations.

No declaration of war accompanies this tunnel. No UN resolution marks the moment. But this is among the most consequential acts of hydraulic statecraft in South Asian history, and Pakistan faces a narrowing window in which to mount an effective response.

The Indus Waters Treaty of 1960 stands as on...