Are you referring to the Battle of Talas? While I do not want to entirely downplay a battle that involved some tens of thousands on either side, in truth it was not much of a war--both the Abbasid and Tang were drawn into the battle due to their local vassals rather than through true conquest intentions, and rather notably no actual territory exchanged hands as a result nor did either side make much of the battle in their own records. Both empires would soon be consumed by more pressing matters, and they both soon established fairly friendly diplomatic relations. As a curio, the battle is truly fascinating--the two great empires of the early medieval period, duking it out!--but its status as a "decisive battle of world history" is likely due in large part to wishful thinking. It is a bit disappointing to think the legendary Tang and Abbasid empires fought a war and that war was not particularly important to either party, but it is hard to argue for it.

There are two ways that some have tried, however. The first is that among the Chinese prisoners were craftsmen who taught the Muslim world the art of papermaking. This is not entirely impossible, even though paper making was known in central Asia for centuries before, it was only in the decades after the battle that widespread papermaking traveled West. But it is also a bit too neat, there is a whiff of Marco Polo bringing noodles to Italy to it. The other, somewhat more nebulous but also more significant claim is that this battle marks the beginning of Islamic central Asia. Again, there is a slight confusion of causes here as only a few years later Tang power in the region would collapse as the empire fell into the confusion of the An Lushan rebellion (a somewhat misunderstood war, but it certainly limited Tang capacity to project power so far west). There is an argument that the battle established Islam as a "religion of victory" among the Turkic people and thus played into their conversion, a process that would have world historic significance, however I am not sure that sort of chronological correlation can be established in a truly satisfying way.

Again, I do not want to entirely downplay the importance of a battle in which tens of thousands fought and thousands lost their lives, but it was certainly not seen as truly significant until its description by modern historians like Vasily Bartold.

My source for much of the Chinese context comes from David Graff's Medieval Chinese Warfare 300-900.