Battle of Talas
The Battle of Talas/怛罗斯之战
大唐玄宗朝与阿拉伯阿拔斯王朝之间的战役，阿拉伯联军（加上吐蕃以及反叛唐帝国的葛逻禄）惨胜。据说，中国的战俘将造纸术传到了中东，继而传遍欧洲。
The Battle of Talas, Battle of Talas River, or Battle of Artlakh (Chinese: 怛羅斯戰役; Arabic: معركة نهر طلاس) was a military engagement between the Arab Abbasid Caliphate along with their ally the Tibetan Empire against the Chinese Tang dynasty, governed at the time by Emperor Xuanzong. In July 751 CE, Tang and Abbasid forces met in the valley of the Talas River to vie for control over the Syr Darya region of central Asia. After several days of stalemate, the Karluks originally allied to the Tang defected to the Abbasids and tipped over the balance of power, resulting in a Tang rout. The defeat marked the end of Tang westward expansion and resulted in Muslim control of Transoxiana （即“中亚河中地区”，包括今乌兹别克斯坦全境和哈萨克斯坦西南部） for the next 400 years. Control of this region was economically beneficial for the Abbasids because it was on the Silk Road.
Historians debate whether or not Chinese prisoners captured in the aftermath of the battle brought paper-making technology to the Middle East, where it eventually spread to Europe.
751年7月-8月（唐玄宗天宝十年）唐安西节度使高仙芝领安西都护府二万汉军，外加盟军拔汗那以及葛逻禄部一万人与阿拔斯王朝（即黑衣大食）呼罗珊总督艾布·穆斯林调集的三万阿拉伯骑兵会战于怛罗斯。葛逻禄人勾结阿拉伯人，阵后偷袭唐军，致使唐军战败。
公元755年唐朝发生内乱安史之乱，西域唐军部分被调入内地平叛。剩余唐军仍然坚守西域数十年。公元790年以后，唐朝才失去西域。
Why is the Battle of Talas historically significant?
The Battle of Talas, a significant encounter between Arab and Chinese forces which took place in central Asia in 751 AD. It brought together two mighty empires, the Abbasid Caliphate and the Tang Dynasty, and although not well known today the battle had profound consequences for the future of both civilisations. The Arabs won the confrontation, but the battle marks the point where the Islamic Empire halted its march eastwards, and the Chinese stopped their expansion to the west. It was also a point of cultural exchange: some historians believe that it was also the moment when the technology of paper manufacture found its way from China to the Western world.
Religion
It marked the beginning of the end of Buddhism as the dmoninant religion in Central Asia. It is almost forgotten now, but at the time, central Asia (modern day Xinjiang and all the Stans) were primarily Buddhist. The Tang were very much a champion of Buddhism in general (some brief spat of respite none withstanding.) their retreat out of central Asia saw the beginning of a steady decline of the faith in the region.
Paper-making Technology
It provides a convenient marker on the timeline to mark the decline of Tang/Chinese influence over central Asia. The battle itself was of little strategic consequence, other than that Chinese POWs transferred paper-making technology into central Asia and its consequences elaborated in Usman Qazi's answer. The amount of Tang troops (10,000 strong) involved in the battle was relatively minor when compared to the entirety of the Tang army (over half a million), and the loss was due to the last-minute unexpected defection of allied Karluks forces. The Tang commander Gao Xianzhi retreated to safety, and immediately began preparations for a counter-offensive. However, that counter-offensive never happened due to the An Lushan Rebellion, which lasting over 7 years with an estimate loss of 36 million people according to Steven Pinker's book The Better Angels of Our Nature. This rebellion/civil war was the fundamental cause to Tang's strategic loss of central Asia. Had it never happened, the Battle of Talas would probably be forgotten as yet another one of hundreds of border conflicts between two Asian empires.
To add to the other answers, the introduction of paper making caused quite an upheaval in the Islamic religion.
Previously, followers of the disciples of the companions of the prophet (Tabi‘ al-Tabi‘in) and their scholarly lineage appeared to rely largely on their memories of what they had had been taught about Islam. The religion was largely an oral tradition. Afterward, there was a flurry of activity in collecting the hadith- accounts of Prophet Muhammad's life and teachings. This is what has ever since driven Islam and defined its internal polemics.
The battle was between the Tang Dynasty and the Karluk Mercenary, under Gao Xianzhi, Li Siye, Duan Xiushi, against the Tibetan Empire and the Abbasid Caliphate, under Ziyad ibn Salid. Depending on the sources, Tang Dynasty had 30,000- 100,000 troops against a force greater than 200,000 (I said greater because we have no clue how many Tibetan were in the fight ). 100,000 and 200,000 were both considered greatly exaggerated. The Tang army consisted of 20,000 Karluk Mercenaries and 10,000 Tang Dynasty troops.
When the battle started the 20,000 Karluk troops betrayed Tang (more like switched sides) and then they started to attack Tang troops in close combat. Also an ally of Tang decided to retreat and never shown up to the battle. Essentially the battle was lost for Tang. However Gao Xianzhi decdied to retreat with only 2,000 troops left. Li Siye managed to inflict heavy casualties to the pursing Arabs. After that Gao wanted to build a new army to defeat the Arabs. However the devastating An Shi Rebellion would occur, meaning that all general and troops were told to put down that rebellion.
This allowed the Abbasid Caliphate to benefit from the wealth from the silk road. This also allowed the Abbasid to continue their conquest of central Asia. Tang prisoners would latter introduce paper making to the Abbasid Caliphate. For Tang the aftermath would be a bloody civil war, which ended their ambition west. The An Shi Rebellion was significant in that it managed to bring over the first Arab communities in China. Their descendent would be the Hui nationality.
https://www.quora.com/Why-is-the-Battle-of-Talas-historically-significant
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READING LIST:
Christopher I. Beckwith, Empires of the Silk Road: A History of Central Eurasia from the Bronze Age to the Present (Princeton University Press, 2011)
Jerry H. Bentley, Old World Encounters: Cross-Cultural Contacts and Exchanges in Pre-Modern Times (Oxford University Press, 1993)
Jonathan Bloom, Paper Before Print: The History and Impact of Paper in the Islamic World (Yale University Press, 2001)
Nicola Di Cosmo (ed.), Warfare in Inner Asian History (500-1800) (Brill, 2001)
Michael R. Drompp, Tang China and the Collapse of the Uighur Empire: A Documentary History (Brill, 2004)
Johan Elverskog, Buddhism and Islam on the Silk Road (University of Pennsylvania Press, 2013)
Valerie Hansen, The Silk Road: A New History (Oxford University Press, 2012)
Hugh Kennedy, The Great Arab Conquests: How The Spread Of Islam Changed The World We Live In (Phoenix Books, 2007)
Xinru Liu, The Silk Road in World History (Oxford University Press, 2010)
Hyunhee Park, Mapping the Chinese and Islamic Worlds: Cross-Cultural Exchange in Pre-Modern Asia (Cambridge University Press, 2012)
Jonathan Karam Skaff, Sui-Tang China and Its Turko-Mongol Neighbors: Culture, Power and Connections, 580-800 (Oxford University Press, 2012)
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