Although the Silk Road was initially formulated during the reign of Emperor Wu of Han (141–87 BCE), it was reopened by the Tang Empire in 639 when Hou Junji conquered the Western Regions, and remained open for almost four decades. It was closed after the Tibetans captured it in 678, but in 699, during Empress Wu's period, the Silk Road reopened when the Tang reconquered the Four Garrisons of Anxi originally installed in 640, once again connecting China directly to the West for land-based trade. The Tang captured the vital route through the Gilgit Valley from Tibet in 722, lost it to the Tibetans in 737, and regained it under the command of the Goguryeo-Korean General Gao Xianzhi.