In foreign affairs an extraordinarily fortunate situation for the Sui dynasty had come into existence. The T'u-chueeh, the Turks, much the strongest people of the north, had given support now to one and now to another of the northern kingdoms, and this, together with their many armed incursions, had made them the dominant political factor in the north. But in the first year of the Sui period (581) they split into two sections, so that the Sui had hopes of gaining influence over them. At first both sections of the Turks had entered into alliance with China, but this was not a sufficient safeguard for the Sui, for one of the Turkish khans was surrounded by Toba who had fled from the vanished state of the Northern Chou, and who now tried to induce the Turks to undertake a campaign for the reconquest of North China. The leader of this agitation was a princess of the Yue-wen family, the ruling family of the Northern Chou. The Chinese fought the Turks several times; but much more effective results were gained by their diplomatic missions, which incited the eastern against the western Turks and vice versa, and also incited the Turks against the Toba clique. In the end one of the sections of Turks accepted Chinese overlordship, and some tribes of the other section were brought over to the Chinese side; also, fresh disunion was sown among the Turks.

Under the emperor Yang Ti, P'ei Chue carried this policy further. He induced the Toeloes tribes to attack the T'u-yue-hun, and then himself attacked the latter, so destroying their power. The T'u-yue-hun were a people living in the extreme north of Tibet, under a ruling class apparently of Hsien-pi origin; the people were largely Tibetan. The purpose of the conquest of the T'u-yue-hun was to safeguard access to Central Asia. An effective Turkestan policy was, however, impossible so long as the Turks were still a formidable power. Accordingly, the intrigues that aimed at keeping the two sections of Turks apart were continued. In 615 came a decisive counter-attack from the Turks. Their khan, Shih-pi, made a surprise assault on the emperor himself, with all his following, in the Ordos region, and succeeded in surrounding them. They were in just the same desperate situation as when, eight centuries earlier, the Chinese emperor had been beleaguered by Mao Tun. But the Chinese again saved themselves by a trick. The young Chinese commander, Li Shih-min, succeeded in giving the Turks the impression that large reinforcements were on the way; a Chinese princess who was with the Turks spread the rumour that the Turks were to be attacked by another tribe--and Shih-pi raised the siege, although the Chinese had been entirely defeated.

In the Sui period the Chinese were faced with a further problem. Korea or, rather, the most important of the three states in Korea, had generally been on friendly terms with the southern state during the period of China's division, and for this reason had been more or less protected from its North Chinese neighbours. After the unification of China, Korea had reason for seeking an alliance with the Turks, in order to secure a new counterweight against China.

A Turco-Korean alliance would have meant for China a sort of encirclement that might have grave consequences. The alliance might be extended to Japan, who had certain interests in Korea. Accordingly the Chinese determined to attack Korea, though at the same time negotiations were set on foot. The fighting, which lasted throughout the Sui period, involved technical difficulties, as it called for combined land and sea attacks; in general it brought little success.