China has long regarded fresh water as a strategic weapon—one that the country’s leaders have no compunction about wielding to advance their foreign-policy goals. After years of using its chokehold ^[1] on almost every major transnational river system in Asia to manipulate water flows themselves, China is now withholding data on upstream flows to put pressure on downstream countries, particularly India.

For decades, China has been dragging its neighbours into high-stakes games of geopolitical poker over water-related issues. Thanks to its forcible annexation of Tibet and other non-Han Chinese ethnic homelands—territories that comprise some 60% of its landmass—China is the world’s unrivaled hydro-hegemon. It is the source of cross-border riparian flows to more countries than any other state.

In recent years, China has worked hard to exploit that status to increase its leverage over its neighbours, relentlessly building upstream dams on international rivers. China is now home to more dams than the rest of the world combined, and the construction continues, leaving downstream neighbours—especially the vulnerable lower Mekong basin states, Nepal, and Kazakhstan—essentially at China’s mercy.

So far, China has refused to enter into a water-sharing treaty with a single country. It does, however, share some hydrological and meteorological data—essential to enable downstream countries to foresee and plan for floods, thereby protecting lives and reducing material losses.

Yet, this year, China decided to withhold such data from India, undermining the efficacy of India’s flood early-warning systems—during Asia’s summer monsoon season, no less. As a result, despite below-normal monsoon rains this year in India’s northeast, through which the Brahmaputra River flows after leaving Tibet and before entering Bangladesh, the region faced unprecedented flooding, with devastating consequences ^[2], especially in Assam state.

China’s decision to withhold crucial data is not only cruel; it also breaches the country’s international obligations. China is one of just three countries that voted against the 1997 United Nations Watercourse Convention ^[3], which called for the regular exchange of hydrological and other data between co-basin states. But China did enter into a five-year bilateral accord ^[4], which expires next year, requiring it to transfer to India hydrological and meteorological data daily from three Brahmaputra-monitoring stations in Tibet during the risky flood season, from 15 May to 15 October. A similar agreement, reached in 2015, covers the Sutlej, another flood-prone river. Both accords arose after flash floods linked to suspected discharges from Chinese projects in Tibet repeatedly ravaged India’s Arunachal and Himachal states.

Unlike some other countries, which offer hydrological data to their downstream counterparts for free, China does so only for a price. (The Watercourse Convention would have required that no charges be levied, unless the data or information was ‘not readily available’—a rule that may also have contributed to China’s ‘no’ vote.)

But it was a price India was willing to pay. And this year, as always, India sent the agreed amount. Yet it received no data, with the Chinese foreign ministry claiming ^[5] after almost four months that upstream stations were being ‘upgraded’ or ‘renovated’. That claim was spurious: China did supply data on the Brahmaputra to Bangladesh ^[6].

Three weeks earlier, the state-controlled newspaper Global Times offered a more plausible explanation for China’s failure to deliver the promised data to India: the data transfer had been intentionally halted ^[7], owing to India’s supposed infringement on Chinese territorial sovereignty in a dispute over the remote Himalayan region of Doklam. For much of the summer, that dispute took the form of a border standoff ^[8] where Bhutan, Tibet, and the Indian state of Sikkim meet.

But even before the dispute flared in mid-June, China was seething over India’s boycott of its 14–15 May summit ^[9] promoting the much-vaunted ‘Belt and Road’ initiative. The denial of data apparently began as an attempt to punish India for condemning ^[10] China’s massive cross-border infrastructure agenda as an opaque, neocolonial enterprise. China’s desire to punish India was then reinforced by the Doklam standoff.

For China, it seems, international agreements stop being binding when they are no longer politically convenient. This reading is reinforced by China’s violations ^[11] of its 1984 pact with the United Kingdom, under which China gained sovereignty over Hong Kong in 1997. China claims that the agreement, based on the formula ‘one country, two systems’, had lost ‘practical significance’ over the last 20 years.

Were the roles reversed, a downstream China would have stridently accused an upstream India of exacerbating flood-related death and destruction by breaching its international obligations. But just as China has unilaterally and aggressively asserted its territorial and maritime claims in Asia, it is using the reengineering of cross-border riparian flows and denial of hydrological data to deepen its regional power.

In fact, China’s cutoff of water data, despite the likely impact on vulnerable civilian communities, sets a dangerous precedent of indifference to humanitarian considerations. It also highlights how China is fashioning unconventional tools of coercive diplomacy, whose instruments already range from informally boycotting goods from a targeted country to halting strategic exports (such as of rare-earth minerals) and suspending Chinese tourist travel.

Now, by seizing control over water—a resource vital to millions of lives and livelihoods—China can hold another country hostage without firing a single shot. In a water-stressed Asia, taming China’s hegemonic ambition is now the biggest strategic challenge.