At an altitude of 3,000 m (9,800 ft), Dromo experiences a wetter and more temperate climate compared to much of Tibet due to its location south of the Himalayas. Vegetation ranges from Eastern Himalayan broadleaf forests to Eastern Himalayan alpine shrub and meadows as elevation increases. The plant Pedicularis chumbica (春丕马先蒿) is named after the valley.
The 1904 Younghusband Expedition of British India traversed Dromo en route to Lhasa. Following the expedition, Britain gained control of the valley as a war indemnity, which was returned to Tibet on February 8, 1908, after China agreed to pay the indemnity in installments.
The Tibetan name, Gromo, is pronounced Dromo or Tromo. In Dzongkha (the language of Bhutan), the valley is known as Gyumo, pronounced J'umo, with "J'umbi" used as an adjective meaning "of or pertaining to [J'umo]". The term "Chumbi" first appeared in the writings of Joseph Dalton Hooker, the first European to document the region. Some sources translate “Chumbi Valley” as Lho Dromo, meaning “southern Dromo,” implying the northern part of the valley around Phari is not always referred to as Chumbi by Tibetans, though in English usage, "Chumbi Valley" encompasses both areas.
Hooker described Dromo as a triangular region bounded to the west by the Chola Range and to the east by the Kamphee or Chakoong range. While these names are not found in later literature, mid-20th century maps identify the eastern border as the Masang Kyungdu Range, with Khundugang peak as a prominent landmark.
Dromo, currently part of the People's Republic of China, lies between the Himalayan states of Sikkim to the west and Bhutan to the east.
The valley holds strategic importance, identified by Susan Walcott as a critical “mountain chokepoint” in global power competition, and by John Garver as “the single most strategically important piece of real estate in the entire Himalayan region”. Dromo points towards India’s Siliguri Corridor, a narrow 24-kilometer-wide strip of land between Nepal and Bangladesh, which connects India’s central regions to its northeast, including the contested state of Arunachal Pradesh. This corridor is often referred to as the “chicken’s neck” due to its vulnerability.
Historical accounts indicate Dromo served as a trade route between Tibet and India as early as the first millennium CE. A lotsawa named Yontendrak was granted control of the route to India by an Indian teacher named Aryadeva, as recorded in 14th-century texts. In the mid-14th century, Phakpa Balzang, a Sakya governor, is said to have constructed a dzong and trade marts at Phari after inviting and subsequently killing chiefs and teachers from Paro and Haa.
In 1206 A.D., Bakhtiyar Khilji’s invasion of Tibet through Dromo was repelled by Tibetan forces. Sikkimese tradition holds that the Kingdom of Sikkim, founded in 1642, initially included Dromo, though historians qualify this claim. Following the unification of Nepal in 1756, Nepal and Bhutan contested Sikkim, with Dromo frequently changing hands. The 1890 Anglo-Chinese treaty established British suzerainty over Sikkim and defined the border with Tibet along the northern watershed of the Teesta River, a boundary that remains in place today.
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