There has been a few discussions on "WI the Chinese won the battle of Talas". And it is generally agreed that it was the Anshi Rebellion that truly destroyed the Tang hegemony in the West. But I would like to raise another possiblity.
The Chinese army performed better than it was in OTL, and the Qarluqs remained in the Chinese side, allowing Gao to achieve a Pyrrhic victory over the Arabs. (Note that given the numerical superiority of the Arabs, it seems a crushing victory could not be achieved.) The battle reassured the Tang hegemony over Transoxiana, and the Abbasids do not make any expansion towards the region in the coming decade or two. But it again clarified that the followers of
Pirooz will have to live in Central Asia forever.
Only four years after the battle at Talas, in 755, An Lushan rebelled in Fanyang (Beijing), and rapidly moved towards Chang'an, the imperial capital. The Emperor called upon all the
jiedushis in the periphery to bring their army back to defend Chang'an. In OTL, Gao and his deputy, Feng Changqing, returned, and later falsely accused of corruption by a eunuch, Bian Lingcheng, and were killed on the frontline.
In this TL, things go a bit different. Some small tribes rebelled against the
Anxi Protectorate General, and Gao has an ideal reason not to bring his army back to China. (The rebellion could be ignited by Gao himself.) Afterwards, Anxi, with its elite troops intact, could become a bastion of Chinese civilization in the West, maintaining peace within its borders, and only paying nominal allegiance to the Tang government. To escape from prolonged civil war in China proper, some Chinese would migrate to Anxi too, instead of going South as in OTL. Also, the presence of Sasanid lineage following Pirooz might also attract some Parsees to migrate to Central Asia, instead of India.
Over time, the Anxi Kingdom (or Duchy, as Gao was duke of Miyun - though it is only a honorific title, resembling more like the modern British titles than contemporaneous European one) might also exert its control over the neighbouring, weaker
Beiting Protectorate General (site in Chinese). It might try to protect Gansu from Tibetan conquest as in OTL, but it would be more likely for it to focus on its internal affairs and look to the West, where the Arabs look always to Anxi jealously.
If the Anxi became strong enough, it might try to 'reconquer' China after hearing Tang's falling (it might be earlier than OTL). It is unlikely to succeed, but it would bring back even more Chinese to the 'Western regions', strengthening Chinese rule in the region, and further depopulate Northern China.
Any ideas on this proposal?