By withholding the tribute which Irene had agreed to pay to Harun al-Rashid, Nicephorus committed himself to a war with the Saracens.
Embassies passed between Charlemagne and Harun in the years 180 (A.D.
At Bagdad, in the reign of Mamun (813-833), the son of Harun al-Rashid, philosophical works were translated by Syrian Christians from Greek into Syriac and from Syriac into Arabic. It was in his reign that Aristotle was first translated into Arabic, and, shortly afterwards, we have Syriac and Arabic renderings of commentators on Aristotle, and of portions of Plato, Hippocrates and Galen; while in the 10th century new translations of Aristotle and his commentators were produced by the Nestorian Christians.
Charlemagne in particular was closely connected with Jerusalem: the patriarch sent him the keys of the city and a standard in Boo; and in 807 Harun al-Rashid recognized this symbolical cession, and acknowledged Charlemagne as protector of Jerusalem and owner of the church of the Sepulchre.
Depending on coloured tiles and gorgeous fabrics for their rich effects, nothing of the buildings of the times of Harun al-Rashid or Mamun, once counted so magnificent, have come down to us.
Half a day's journey beyond Sura, on the Mesopotamian side of the river, are the extensive ruins of Haragla (Heraclea) and Rakka, once the capital of Harun al-Rashid (Nicephorium of Alexander; Callinicus of the Seleucids and Romans).
In the reign of Harun al-Rashid disturbances broke out in Khorasan which, were temporarily appeased by a visit from Harun himself.
A proposition by the cadi Abu Yusuf to Harun al-Rashid to renew it had not been adopted.
Already under Charlemagne this development is noticeable; in his generous treatment of the Jews this Christian emperor stood in marked contrast to his contemporary the caliph Harun al-Rashid, who persecuted Jews and Christians with equal vigour.
It is believed that this mina divided by 12 unciae by the Romans is the origin of the Arabic ratl of 12 ukiyas, or 5500 grains (33), which is said to have been sent by Harun al-Rashid to Charlemagne, and so to have originated the French monetary pound of 5666 grains.