Task: Question Generation
Input (English): Passage: Nathu La is located on the 563 km (350 mi) Old Silk Route, an offshoot of the historic Silk Road. The Old Silk Route connects Lhasa in Tibet to the plains of Bengal to the south. In 1815, trade volume increased after the British annexed territories belonging to the Sikkimese, Nepalese, and Bhutanese. The potential of Nathu La was realised in 1873, after the Darjeeling Deputy Commissioner published a report on the strategic importance of mountain passes between Sikkim and Tibet. In December 1893, the Sikkimese monarchy and Tibetan rulers signed an agreement to increase trade between the two nations. The agreement culminated in 1894 when the trade pass was opened.
Nathu La played a vital role in the 1903–1904 British expedition to Tibet, which sought to prevent the Russian Empire from interfering in Tibetan affairs and thus gaining a foothold in the region. In 1904, Major Francis Younghusband, serving as the British Commissioner to Tibet, led a successful mission through Nathu La to capture Lhasa. This led to the setting up of trading posts at Gyantse and Gartok in Tibet, and gave control of the surrounding Chumbi Valley to the British. The following November, China and Great Britain ratified an agreement approving trade between Sikkim and Tibet.
In 1947 and 1948, a popular vote for Sikkim to join newly independent India failed and Indian Prime Minister Jawaharlal Nehru agreed to a special protectorate status for Sikkim. Sikkim agreed to be a protectorate nation and Indian troops were allowed to man its borders, including Nathu La. During this period, more than 1,000 mules and 700 people were involved in cross-border trade through Nathu La. In 1949, when the Tibetan government expelled the Chinese living there, most of the displaced Chinese returned home through the Nathu La–Sikkim–Kolkata route.The current Dalai Lama, Tenzin Gyatso, used this pass to travel to India for the 2,500th birthday celebration of Gautama Buddha, which was held between November 1956 and February 1957. Later, on 1 September 1958, Nehru, his daughter Indira Gandhi, and Palden Thondup Namgyal (son of—and internal affairs adviser to—Tashi Namgyal, the Chogyal of Sikkim) used this pass to travel to nearby Bhutan.
After the People's Republic of China took control of Tibet in 1950 and suppressed a Tibetan uprising in 1959, the passes into Sikkim became a conduit for refugees from Tibet. During the 1962 Sino-Indian War, Nathu La witnessed skirmishes between soldiers of the two countries. Shortly thereafter, the passage was sealed and remained closed for more than four decades. Between 7 and 13 September 1967, China's People's Liberation Army and the Indian Army had six-day "border skirmishes", including the exchange of heavy artillery fire. In 1975, Sikkim acceded to India and Nathu La became part of Indian territory. China, however, refused to acknowledge the accession at that time.
Output (English): What war occurred right before the passage was sealed for 4 decades?