The Chinese war Military_operation against the Western Turks began Process_start in 640 with the annexation Change_of_leadership of the Tarim Basin oasis state Gaochang , an ally Collaboration of the Western Turks .

Several of the oasis states had once been vassals of the Tang Dynasty , but switched Change their allegiance to the Western Turks when they grew Coming_to_be suspicious Know of the military ambitions of the Tang .