The dominions of the emperor Tai-tsung (627), the second Tang monarch, extended southward into Annam and westward to the Caspian Sea. His southern frontier in that direction marched with that of Persia. His northern ran along the Altai from the Kirghis steppe, north of the desert of Gobi. But it did not include Korea, which was conquered and made tributary by his son. This Tang dynasty civilized and incorporated into the Chinese race the whole of the southward population, and just as the Chinese of the north call themselves the "men of Han," so the Chinese of the south call themselves the "men of Tang." The law was codified, the literary examination system was revised, and a complete and accurate edition of all the Chinese classics was produced. To the court of Tai-tsung came an embassy from Byzantium, and, what is more significant, from Persia came a company of Nestorian missionaries (631). These latter Tai-tsung received with great respect; he heard them state the chief articles of their creed, and ordered the Christian scriptures to be translated into Chinese for his further examination. In 638 he announced that he found the new religion entirely satisfactory, and that it might be preached within the empire. He also allowed the building of a church and the foundation of a monastery.
A still more remarkable embassy also came to the court of Tai-tsung in the year 628, three years earlier than the Nestorians. This was a party of Arabs, who came by sea to Canton in a trading vessel from Yanbu, the port of Medina in Arabia. (Incidentally it is interesting to know that there were such vessels engaged in an east and west trade at this time.) These Arabs had been sent by that Muhammad we have already mentioned, who styled himself "The Prophet of God," and the message they brought to Tai-tsung was probably identical with the summons which was sent in the same year to the Byzantine emperor Heraclius and to Kavadh in Ctesiphon. But the Chinese monarch neither neglected the message as Heraclius did, nor insulted the envoys after the fashion of the parricide Kavadh. He received them well, expressed great interest in their theological views, and assisted them, it is said, to build a mosque for the Arab traders in Canton—a mosque which survives to this day. It is one of the oldest mosques in the world.