China has begun building what it says will be the world’s largest hydropower dam on the Yarlung Zangbo river in Tibet, just upstream of where the waterway enters India as the Siang and later the Brahmaputra. A confidential Indian government analysis seen by Reuters estimates the Chinese structure could divert about 40 billion cubic meters of water and cut trans-boundary flows by as much as 85 % during the dry season. New Delhi views the project as a strategic threat to the water security of more than 100 million people in India and Bangladesh. Prime Minister Narendra Modi’s office has therefore accelerated planning for the Upper Siang Multipurpose Storage Dam in Arunachal Pradesh, which would be India’s largest, with a planned storage capacity of 14 billion cubic meters. Senior officials held a series of meetings this year to fast-track surveys, moving equipment under armed guard amid sometimes violent local resistance. The Chinese foreign ministry says its dam will not harm downstream countries and has undergone rigorous environmental studies. India has raised the issue diplomatically while pressing ahead with its own project, which officials argue could release water during shortages and buffer sudden surges from the Chinese facility. Even so, the Indian dam faces prolonged construction timelines, seismic risks and opposition from indigenous communities that fear displacement.