The necessity for this military reform was clearly shown by the experience of his first campaign with these implacable enemies, when, in the year of his accession and before his organization had been completed, a horde of these barbarians broke into the empire and carried all before them, almost to the gates of the capital. On this occasion Taitsong resorted to diplomacy and remonstrance. He rode almost unattended to the Tartar camp, and reproached their chiefs with their breach of faith, reminding them that on his sending one of his sisters to be the bride of their chief they had sworn by a solemn oath to keep the peace. He asked: "Are these proceedings worthy, I will not say of princes, but of men possessing the least spark of honor? If they forget the benefits they have received from me, at the least they ought to be mindful of their oaths. I had sworn a peace with them; they are now violating it, and by that they place the justice of the question on my side." The Chinese chroniclers declare that the Tartars were so impressed by Taitsong's majestic air and remonstrances that they agreed to retire, and fresh vows of friendship and peace were sworn over the body of a white horse at a convention concluded on the Pienkiao bridge across the Weichoui River. The only safe deduction from this figurative narrative is that there was a Tartar incursion, and that the Chinese army did not drive back the invaders. Their retreat was probably purchased, but it was the first and last occasion on which Taitsong stooped to such a measure.

The peace of Pienkiao was soon broken. The tribes again drew their forces to a head for the purpose of invading China, but before their plans were complete Taitsong anticipated them by marching into their territory at the head of a large army. Taken by surprise, the Tartars offered but a feeble resistance. Several of their khans surrendered, and at a general assembly Taitsong proclaimed his intention to govern them as Khan of their khans, or by the title of Tien Khan, which means Celestial Ruler. This was the first occasion on which a Chinese ruler formally took over the task of governing the nomad tribes and of treating their chiefs as his lieutenants. Down to the present day the Chinese emperor continues to govern the Mongol and other nomadic tribes under this very title, which the Russians have rendered as Bogdo Khan. The success of this policy was complete, for not only did it give tranquillity to the Chinese borders, but it greatly extended Chinese authority. Kashgaria was then, for the first time, formed into a province under the name of Lonugsi, and Lichitsi, one of the emperor's best generals, was appointed Warden of the Western Marches. Some of the most influential of Taitsong's advisers disapproved of this advanced policy, and attempted to thwart it, but in vain. Carried out with the vigor and consistency of Taitsong there cannot be two opinions about its wisdom and efficacy.

During this reign the relations between China and two of its neighbors, Tibet and Corea, were greatly developed, and the increased intercourse was largely brought about by the instrumentality of war. The first envoys from Tibet, or, as it was then called, Toufan or Toupo, are reported to have reached the Chinese capital in the year 634. At that time the people of Tibet were rude and unlettered, and their chiefs were little better than savages. Buddhism had not taken that firm hold on the popular mind which it at present possesses, and the power of the lamas had not arisen in what is now the most priest-ridden country in the world. A chief, named the Sanpou - which means the brave lord - had, about the time of which we are speaking, made himself supreme throughout the country, and it was said that he had crossed the Himalaya and carried his victorious arms into Central India. Curiosity, or the desire to wed a Chinese princess, and thus to be placed on what may be termed a favored footing, induced the Sanpou to send his embassy to Singan; but although the envoys returned laden with presents, Taitsong declined to trust a princess of his family in a strange country and among an unknown people. The Sanpou chose to interpret this refusal as an insult to his dignity, and he declared war with China. But success did not attend his enterprise, for he was defeated in the only battle of the war, and glad to purchase peace by paying five thousand ounces of gold and acknowledging himself a Chinese vassal. The Sanpou also agreed to accept Chinese education, and as his reward Taitsong gave him one of his daughters as a wife. It is stated that one of his first reforms was to abolish the national practice of painting the face, and he also built a walled city to proclaim his glory as the son-in-law of the Emperor of China. During Taitsong's life there was no further trouble on the side of Tibet.