"In reply to the Tibetan embassy, Emperor Wu [Zetian] sent a remarkably astute diplomat-general, Kuo Yuan-chen. When Mgar [Tongtsen Yulsung] met with him he demanded that the Chinese abandon the Four Garrisons, and that the nations of the Tarim and of the Western Turks be freed to live under their own rulers. Kuo objected that the Tibetans were a different race from the Four Garrisons (in other words, the nations kept under Chinese domination by the Four Garrisons) and the Turks. He asked Mgar, "How could you not have the intention of annexing them?" One can almost see the innocent look on Mgar's face as he replied according to Chinese sources: "If Tibet lusted after territory, we would trouble your borders, and thus invade (your prefectures of) Kan and Liang; why should we wish to scheme for profit over 10,000 Li away?" Mgar further pointed out that the Western Turks were "extremely far away from China," but that although their eastern five tribes, the Tardus, were close to the T'ang's Pacified West, the western five tribes, the Nu-shih-pi, were only separated from Tibet by a desert, and so their raiders could reach Tibet very quickly. Thus, Mgar concluded, the disposition of the Western Turks was a matter of concern for Tibet. Kuo gave no answer but returned to court along with a Tibetan envoy to report Mgar's proposal to Emperor Wu."
The Tibetan Empire in Central Asia
Christopher I. Beckwith
Pg.58-59