Why did Yongle order the construction?
Tsai writes that shortly after the visit by Deshin Shekpa, the Yongle Emperor ordered the construction of a road and of trading posts in the upper reaches of the Yangzi and Mekong Rivers in order to facilitate trade with Tibet in tea, horses, and salt. The trade route passed through Sichuan and crossed Shangri-La County in Yunnan. Wang and Nyima assert that this "tribute-related trade" of the Ming exchanging Chinese tea for Tibetan horses—while granting Tibetan envoys and Tibetan merchants explicit permission to trade with Han Chinese merchants—"furthered the rule of the Ming dynasty court over Tibet". Rossabi and Sperling note that this trade in Tibetan horses for Chinese tea existed long before the Ming. Peter C. Perdue says that Wang Anshi (1021–1086), realizing that China could not produce enough militarily capable steeds, had also aimed to obtain horses from Inner Asia in exchange for Chinese tea. The Chinese needed horses not only for cavalry but also as draft animals for the army's supply wagons. The Tibetans required Chinese tea not only as a common beverage but also as a religious ceremonial supplement. The Ming government imposed a monopoly on tea production and attempted to regulate this trade with state-supervised markets, but these collapsed in 1449 due to military failures and internal ecological and commercial pressures on the tea-producing regions.