The Turkestan wars began in 639 with an attack on the city-state of Kao-ch'ang (Khocho). This state had been on more or less friendly terms with North China since the Toba period, and it had succeeded again and again in preserving a certain independence from the Turks. Now, however, Kao-ch'ang had to submit to the western Turks, whose power was constantly increasing. China made that submission a pretext for war. By 640 the whole basin of Turkestan was brought under Chinese dominance. The whole campaign was really directed against the western Turks, to whom Turkestan had become subject. The western Turks had been crippled by two internal events, to the advantage of the Chinese: there had been a tribal rising, and then came the rebellion and the rise of the Uighurs (640-650). These events belong to Turkish history, and we shall confine ourselves here to their effects on Chinese history. The Chinese were able to rely on the Uighurs; above all, they were furnished by the Toeloes Turks with a large army, with which they turned once more against Turkestan in 647-48, and now definitely established their rule there.

The active spirit at the beginning of the T'ang rule had not been the emperor but his son Li Shih-min, who was not, however, named as heir to the throne because he was not the eldest son. The result of this was tension between Li Shih-min and his father and brothers, especially the heir to the throne. When the brothers learned that Li Shih-min was claiming the succession, they conspired against him, and in 626, at the very moment when the western Turks had made a rapid incursion and were once more threatening the Chinese capital, there came an armed collision between the brothers, in which Li Shih-min was the victor. The brothers and their families were exterminated, the father compelled to abdicate, and Li Shih-min became emperor, assuming the name T'ai Tsung (627-649). His reign marked the zenith of the power of China and of the T'ang dynasty. Their inner struggles and the Chinese penetration of Turkestan had weakened the position of the Turks; the reorganization of the administration and of the system of taxation, the improved transport resulting from the canals constructed under the Sui, and the useful results of the creation of great administrative areas under strong military control, had brought China inner stability and in consequence external power and prestige. The reputation which she then obtained as the most powerful state of the Far East endured when her inner stability had begun to deteriorate. Thus in 638 the Sassanid ruler Jedzgerd sent a mission to China asking for her help against the Arabs. Three further missions came at intervals of a good many years. The Chinese declined, however, to send a military expedition to such a distance; they merely conferred on the ruler the title of a Chinese governor; this was of little help against the Arabs, and in 675 the last ruler, Peruz, fled to the Chinese court.

The last years of T'ai Tsung's reign were filled with a great war against Korea, which represented a continuation of the plans of the Sui emperor Yang Ti. This time Korea came firmly into Chinese possession. In 661, under T'ai Tsung's son, the Korean fighting was resumed, this time against Japanese who were defending their interests in Korea. This was the period of great Japanese enthusiasm for China. The Chinese system of administration was copied, and Buddhism was adopted, together with every possible element of Chinese culture. This meant increased trade with Japan, bringing in large profits to China, and so the Korean middleman was to be eliminated.

T'ai Tsung's son, Kao Tsung (650-683), merely carried to a conclusion what had been begun. Externally China's prestige continued at its zenith. The caravans streamed into China from western and central Asia, bringing great quantities of luxury goods. At this time, however, the foreign colonies were not confined to the capital but were installed in all the important trading ports and inland trade centres. The whole country was covered by a commercial network; foreign merchants who had come overland to China met others who had come by sea. The foreigners set up their own counting-houses and warehouses; whole quarters of the capital were inhabited entirely by foreigners who lived as if they were in their own country. They brought with them their own religions: Manichaeism, Mazdaism, and Nestorian Christianity. The first Jews came into China, apparently as dealers in fabrics, and the first Arabian Mohammedans made their appearance. In China the the foreigners bought silkstuffs and collected everything of value that they could find, especially precious metals. Culturally this influx of foreigners enriched China; economically, as in earlier periods, it did not; its disadvantages were only compensated for a time by the very beneficial results of the trade with Japan, and this benefit did not last long.