Evidence from documents on woodslips found near modern-day Hotan shows that "Tibetan armies, including previously subjugated Sumpa and Zhangzhung elements" were stationed along the Southern Silk Route from the mid-eighth to mid-ninth century CE.
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Their territory was absorbed by the Tibetan Empire in the late 7th century, after which point they gradually lost their independent identity.
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The Old Tibetan Chronicle says that Myang Mangporje advised Songtsän Gampo against attacking the Sumpa, who had been among his father Namri Songtsen’s feudatory states.