Allied troops from the Ferghana valley deserted the loser during that battle between these two empires, the 751 Battle of Talas.
Arthur Waley conjectured that a defeat of one of these two powers at the hands of the other inspired the poem "Fighting South of the Ramparts." A physician in one of these empires named Rhazes gave an account of a student from the other copying notes in "grass script." A prisoner from one of these two empires held by the other wrote a "Record of Travels" which gives the capital of the Dashi as Kufa. A battle between these two empires was decided by the defection of Karluk mercenaries from the side of general Goseonji to the other. According to (*) al-Tabari, a battle between these two empires led to the spread of paper-making to the west. Allied troops from the Ferghana valley deserted the loser during that battle between these two empires, the 751 Battle of Talas. For 10 points, name these two powers, one of which had a ruler named Abu Jafar who send the other some mercenaries to help put down the An Lushan rebellion.
Arthur Waley conjectured that a defeat of one of these two powers at the hands of the other inspired the poem "Fighting South of the Ramparts."
Abbasid Caliphate and Tang dynasty [accept China or Chinese empire in place of "Tang"; prompt on Islamic world or Arabs or Muslims; accept Tibet in place of Abbasids until "Dashi" due to ambiguity]