You'll end up with Sinicized pagan/Taoist/Buddhist Turks from China to Eastern Europe instead of Muslim Turks we see today.
A longer lasting Tang Empire which could have emerged even more powerful than it was historically. This would change how the China we saw at the beginning of the colonial period. Possibly a China that would not be inward looking since the Tang, like the earlier Han, was imperialistic, expansionist and muliticultural. In fact, China itself would be fundamentally different culturally.
Chinese garrisons, allies and protectorates all along the Silk Road could actually reach the gates of Europe itself. Chinese troops approaching the border of the Byzantine Empire. The Byzantine Empire might or might be overrun by Sinicized Turks.
Zorastrian Persia might have been restored by the Sassinid Princes residing in Tang China.
The biggest change in history might have been no Genghiz Khan or an Mongolian empire if the Tang Armies remained powerful and dominant in the northern borderlands.
Then again even after a win at Talas, the Tang could have fallen. The problem with the Tang was it was overstretched and too many of the Imperial Armies along the borders saw themselves as a separate kingdoms with claim to the Empire.
An Lushan was from an Persian protectorate of the Tang, Sogdia. His Imperial Tang Army that was supposed to protect the northeast turned on the capitol. The Uighur Turks, a Sinicized tribe allied to the Tang on Mongolian border fought An Lushan for the Tang royal house. Other Imperial armies and protectorates fought for and against the Empire.
Remember, it wasn't the Battle of Talas which brought down the Empire. It was the An Lushan Rebellion that destroyed central power. If the An Lushan rebellion didn't take place, the Tang would have attempted to retake Central Asia and it might been successful.
Actually, the large number of foreign troops in Kao's army (40,000 were allies with only 20,000 Han Chinese) already indicated that the Empire was overstretched.
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