[A-Z April Blogging] [X] Xīyù 西域
Under the Táng, Xīyù was everything beyond the city of Dūnhuáng (燉煌) in Gānsù and the nearby Jade Pass (Yùmén Guān 玉門關): the 'Western Regions', i.e. the oases of the Tarim Basin inhabited by non-Hàn people but under the suzerainty of China; or Central Asia in its entirety; or even anything west of the Jade Pass, and most notably India, as in the Great Táng Records on the Western Regions (Dà Táng Xīyù Jì 大唐西域記), the travelogue written by the 7th century Buddhist monk Xuánzàng (玄奘).
After the Ān Lùshān Rebellion (755-763), the Táng lost control of the 'Western Regions', which became alternatively controlled by the Tibetans, independent city-states, or controlled by local warlords (of various ethnicities).
Under the Southern Sòng, the 'Western Regions' were controlled by the Qìdān.
Under the Yuán, the 'Western Regions' were conquered by the Mongols, like almost all the other provinces covered by The Celestial Empire. The resulting 'Pax Mongolica' brought prosperity and safety of travel to the Silk Road. Xīyù became a significant cultural and trade conduit between East Asia, the Indian subcontinent, the Islamic world and Europe.
Under the Míng, there was an initial effort of expansion and re-conquest towards Xīyù that was halted at the disastrous Battle of Tŭmù, after which the Míng Dynasty started the isolationist politics for which it's remembered; the Great Wall of China was built under the Míng. The Silk Road was abandoned and replaced with sea trade routes. After the Battle of Tŭmù, the Western Regions became alternatively controlled by the Tibetans and various Mongol tribes. They were retaken under the Qīng.